diff --git a/src/auth-service/auth.rs b/src/auth-service/auth.rs
new file mode 100644
index 0000000..390e7be
--- /dev/null
+++ b/src/auth-service/auth.rs
@@ -0,0 +1,254 @@
+use std::sync::Mutex;
+
+use crate::{sessions::Sessions, users::Users};
+
+use tonic::{Request, Response, Status};
+
+use authentication::auth_server::Auth;
+use authentication::{
+    SignInRequest, SignInResponse, SignOutRequest, SignOutResponse, SignUpRequest, SignUpResponse,
+    StatusCode,
+};
+
+pub mod authentication {
+    tonic::include_proto!("authentication");
+}
+
+// Re-exporting
+pub use authentication::auth_server::AuthServer;
+pub use tonic::transport::Server;
+
+pub struct AuthService {
+    users_service: Box<Mutex<dyn Users + Send + Sync>>,
+    sessions_service: Box<Mutex<dyn Sessions + Send + Sync>>,
+}
+
+impl AuthService {
+    pub fn new(
+        users_service: Box<Mutex<dyn Users + Send + Sync>>,
+        sessions_service: Box<Mutex<dyn Sessions + Send + Sync>>,
+    ) -> Self {
+        Self {
+            users_service,
+            sessions_service,
+        }
+    }
+}
+
+#[tonic::async_trait]
+impl Auth for AuthService {
+    async fn sign_in(
+        &self,
+        request: Request<SignInRequest>,
+    ) -> Result<Response<SignInResponse>, Status> {
+        println!("Got a request: {:?}", request);
+
+        let req = request.into_inner();
+
+        // Get user's uuid from `users_service`. Panic if the lock is poisoned.
+
+        // Match on `result`. If `result` is `None` return a SignInResponse with a the `status_code` set to `Failure`
+        // and `user_uuid`/`session_token` set to empty strings.
+        self.users_service
+            .lock()
+            .expect("failed to get lock on users_service")
+            .get_user_uuid(req.username, req.password)
+            .map_or_else(
+                || {
+                    Ok(Response::new(SignInResponse {
+                        status_code: StatusCode::Failure.into(),
+                        user_uuid: String::new(),
+                        session_token: String::new(),
+                    }))
+                },
+                |user_uuid| {
+                    let session_token: String = self
+                        .sessions_service
+                        .lock()
+                        .expect("failed to get lock on sessions_service")
+                        .create_session(&user_uuid);
+
+                    Ok(Response::new(SignInResponse {
+                        status_code: StatusCode::Success.into(),
+                        user_uuid,
+                        session_token,
+                    }))
+                },
+            )
+    }
+
+    async fn sign_up(
+        &self,
+        request: Request<SignUpRequest>,
+    ) -> Result<Response<SignUpResponse>, Status> {
+        println!("Got a request: {:?}", request);
+
+        let req = request.into_inner();
+
+        // let result: Result<(), String> = todo!(); // Create a new user through `users_service`. Panic if the lock is poisoned.
+
+        self.users_service
+            .lock()
+            .expect("failed to get lock on users_service")
+            .create_user(req.username, req.password)
+            .map_or_else(
+                |_| {
+                    Ok(Response::new(SignUpResponse {
+                        status_code: StatusCode::Failure.into(),
+                    }))
+                },
+                |()| {
+                    Ok(Response::new(SignUpResponse {
+                        status_code: StatusCode::Success.into(),
+                    }))
+                },
+            )
+    }
+
+    async fn sign_out(
+        &self,
+        request: Request<SignOutRequest>,
+    ) -> Result<Response<SignOutResponse>, Status> {
+        println!("Got a request: {:?}", request);
+
+        let req = request.into_inner();
+
+        // TODO: Delete session using `sessions_service`.
+        self.sessions_service
+            .lock()
+            .expect("failed to get lock on sessions_service")
+            .delete_session(&req.session_token);
+
+        let reply: SignOutResponse = SignOutResponse {
+            status_code: StatusCode::Success.into(),
+        }; // Create `SignOutResponse` with `status_code` set to `Success`
+
+        Ok(Response::new(reply))
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::{sessions::SessionsImpl, users::UsersImpl};
+
+    use super::*;
+
+    #[tokio::test]
+    async fn sign_in_should_fail_if_user_not_found() {
+        let users_service = Box::new(Mutex::new(UsersImpl::default()));
+        let sessions_service = Box::new(Mutex::new(SessionsImpl::default()));
+
+        let auth_service = AuthService::new(users_service, sessions_service);
+
+        let request = tonic::Request::new(SignInRequest {
+            username: "123456".to_owned(),
+            password: "654321".to_owned(),
+        });
+
+        let result = auth_service.sign_in(request).await.unwrap().into_inner();
+
+        assert_eq!(result.status_code, StatusCode::Failure.into());
+        assert_eq!(result.user_uuid.is_empty(), true);
+        assert_eq!(result.session_token.is_empty(), true);
+    }
+
+    #[tokio::test]
+    async fn sign_in_should_fail_if_incorrect_password() {
+        let mut users_service = UsersImpl::default();
+
+        let _ = users_service.create_user("123456".to_owned(), "654321".to_owned());
+
+        let users_service = Box::new(Mutex::new(users_service));
+        let sessions_service = Box::new(Mutex::new(SessionsImpl::default()));
+
+        let auth_service = AuthService::new(users_service, sessions_service);
+
+        let request = tonic::Request::new(SignInRequest {
+            username: "123456".to_owned(),
+            password: "wrong password".to_owned(),
+        });
+
+        let result = auth_service.sign_in(request).await.unwrap().into_inner();
+
+        assert_eq!(result.status_code, StatusCode::Failure.into());
+        assert_eq!(result.user_uuid.is_empty(), true);
+        assert_eq!(result.session_token.is_empty(), true);
+    }
+
+    #[tokio::test]
+    async fn sign_in_should_succeed() {
+        let mut users_service = UsersImpl::default();
+
+        let _ = users_service.create_user("123456".to_owned(), "654321".to_owned());
+
+        let users_service = Box::new(Mutex::new(users_service));
+        let sessions_service = Box::new(Mutex::new(SessionsImpl::default()));
+
+        let auth_service = AuthService::new(users_service, sessions_service);
+
+        let request = tonic::Request::new(SignInRequest {
+            username: "123456".to_owned(),
+            password: "654321".to_owned(),
+        });
+
+        let result = auth_service.sign_in(request).await.unwrap().into_inner();
+
+        assert_eq!(result.status_code, StatusCode::Success.into());
+        assert_eq!(result.user_uuid.is_empty(), false);
+        assert_eq!(result.session_token.is_empty(), false);
+    }
+
+    #[tokio::test]
+    async fn sign_up_should_fail_if_username_exists() {
+        let mut users_service = UsersImpl::default();
+
+        let _ = users_service.create_user("123456".to_owned(), "654321".to_owned());
+
+        let users_service = Box::new(Mutex::new(users_service));
+        let sessions_service = Box::new(Mutex::new(SessionsImpl::default()));
+
+        let auth_service = AuthService::new(users_service, sessions_service);
+
+        let request = tonic::Request::new(SignUpRequest {
+            username: "123456".to_owned(),
+            password: "654321".to_owned(),
+        });
+
+        let result = auth_service.sign_up(request).await.unwrap();
+
+        assert_eq!(result.into_inner().status_code, StatusCode::Failure.into());
+    }
+
+    #[tokio::test]
+    async fn sign_up_should_succeed() {
+        let users_service = Box::new(Mutex::new(UsersImpl::default()));
+        let sessions_service = Box::new(Mutex::new(SessionsImpl::default()));
+
+        let auth_service = AuthService::new(users_service, sessions_service);
+
+        let request = tonic::Request::new(SignUpRequest {
+            username: "123456".to_owned(),
+            password: "654321".to_owned(),
+        });
+
+        let result = auth_service.sign_up(request).await.unwrap();
+
+        assert_eq!(result.into_inner().status_code, StatusCode::Success.into());
+    }
+
+    #[tokio::test]
+    async fn sign_out_should_succeed() {
+        let users_service = Box::new(Mutex::new(UsersImpl::default()));
+        let sessions_service = Box::new(Mutex::new(SessionsImpl::default()));
+
+        let auth_service = AuthService::new(users_service, sessions_service);
+
+        let request = tonic::Request::new(SignOutRequest {
+            session_token: "".to_owned(),
+        });
+
+        let result = auth_service.sign_out(request).await.unwrap();
+
+        assert_eq!(result.into_inner().status_code, StatusCode::Success.into());
+    }
+}
diff --git a/src/auth-service/main.rs b/src/auth-service/main.rs
index 28d5022..e2f362a 100644
--- a/src/auth-service/main.rs
+++ b/src/auth-service/main.rs
@@ -1,3 +1,30 @@
-fn main() {
-    println!("auth service");
+use std::sync::Mutex;
+
+mod auth;
+mod sessions;
+mod users;
+
+use auth::*;
+use sessions::{SessionsImpl, Sessions};
+use users::{UsersImpl, Users};
+
+#[tokio::main]
+async fn main() -> Result<(), Box<dyn std::error::Error>> {
+    // Here we are using ip 0.0.0.0 so the service is listening on all the configured network interfaces. This is needed for Docker to work, which we will add later on.
+    // See: https://stackoverflow.com/questions/39525820/docker-port-forwarding-not-working
+    // Port 50051 is the recommended gRPC port.
+    let addr = "[::0]:50051".parse()?;
+
+    let users_service: Box<Mutex<dyn Users + Send + Sync + 'static>> = todo!(); // Create user service instance
+    let sessions_service: Box<Mutex<dyn Sessions + Send + Sync + 'static>> = todo!(); //Create session service instance
+
+    let auth_service = AuthService::new(users_service, sessions_service);
+
+    // Instantiate gRPC server
+    Server::builder()
+        .add_service(AuthServer::new(auth_service))
+        .serve(addr)
+        .await?;
+
+    Ok(())
 }
diff --git a/src/auth-service/sessions.rs b/src/auth-service/sessions.rs
new file mode 100644
index 0000000..699d99f
--- /dev/null
+++ b/src/auth-service/sessions.rs
@@ -0,0 +1,53 @@
+use std::collections::HashMap;
+
+use uuid::Uuid;
+
+pub trait Sessions {
+    fn create_session(&mut self, user_uuid: &str) -> String;
+    fn delete_session(&mut self, user_uuid: &str);
+}
+
+#[derive(Default)]
+pub struct SessionsImpl {
+    uuid_to_session: HashMap<String, String>,
+}
+
+impl Sessions for SessionsImpl {
+    fn create_session(&mut self, user_uuid: &str) -> String {
+        let session: String = Uuid::new_v4().to_string(); // Create a new session using Uuid::new_v4().
+
+        self.uuid_to_session
+            .insert(user_uuid.to_owned(), session.clone());
+
+        session
+    }
+
+    fn delete_session(&mut self, user_uuid: &str) {
+        self.uuid_to_session.remove(user_uuid);
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn should_create_session() {
+        let mut session_service = SessionsImpl::default();
+        assert_eq!(session_service.uuid_to_session.len(), 0);
+        let session = session_service.create_session("123456");
+        assert_eq!(session_service.uuid_to_session.len(), 1);
+        assert_eq!(
+            session_service.uuid_to_session.get("123456").unwrap(),
+            &session
+        );
+    }
+
+    #[test]
+    fn should_delete_session() {
+        let mut session_service = SessionsImpl::default();
+        session_service.create_session("123456");
+        session_service.delete_session("123456");
+        assert_eq!(session_service.uuid_to_session.len(), 0);
+    }
+}
diff --git a/src/auth-service/users.rs b/src/auth-service/users.rs
new file mode 100644
index 0000000..7e2c09c
--- /dev/null
+++ b/src/auth-service/users.rs
@@ -0,0 +1,152 @@
+use pbkdf2::{
+    password_hash::{PasswordHash, PasswordHasher, PasswordVerifier, SaltString},
+    Pbkdf2,
+};
+use rand_core::OsRng;
+use uuid::Uuid;
+
+use std::collections::HashMap;
+
+pub trait Users {
+    fn create_user(&mut self, username: String, password: String) -> Result<(), String>;
+    fn get_user_uuid(&self, username: String, password: String) -> Option<String>;
+    fn delete_user(&mut self, user_uuid: String);
+}
+
+#[derive(Clone)]
+pub struct User {
+    user_uuid: String,
+    username: String,
+    password: String,
+}
+
+impl User {
+    pub fn new(username: String, hashed_password: String) -> Self {
+        let user_uuid = Uuid::new_v4().to_string();
+        Self {
+            user_uuid,
+            username,
+            password: hashed_password,
+        }
+    }
+}
+
+#[derive(Default)]
+pub struct UsersImpl {
+    uuid_to_user: HashMap<String, User>,
+    username_to_user: HashMap<String, User>,
+}
+
+impl Users for UsersImpl {
+    fn create_user(&mut self, username: String, password: String) -> Result<(), String> {
+        if self.username_to_user.contains_key(&username) {
+            static USER_ALREADY_EXISTS: &str = "username is already taken";
+            return Err(USER_ALREADY_EXISTS.to_owned());
+        }
+
+        let salt = SaltString::generate(&mut OsRng);
+
+        let hashed_password = Pbkdf2
+            .hash_password(password.as_bytes(), &salt)
+            .map_err(|e| format!("Failed to hash password.\n{e:?}"))?
+            .to_string();
+
+        let user: User = User::new(username, hashed_password); // Create new user with unique uuid and hashed password.
+
+        self.uuid_to_user
+            .insert(user.user_uuid.clone(), user.clone());
+        self.username_to_user.insert(user.username.clone(), user);
+
+        Ok(())
+    }
+
+    fn get_user_uuid(&self, username: String, password: String) -> Option<String> {
+        let user: &User = self.username_to_user.get(&username)?; // Retrieve `User` or return `None` is user can't be found.
+
+        // Get user's password as `PasswordHash` instance.
+        let hashed_password = user.password.clone();
+        let parsed_hash = PasswordHash::new(&hashed_password).ok()?;
+
+        // Verify passed in password matches user's password.
+        Pbkdf2
+            .verify_password(password.as_bytes(), &parsed_hash)
+            .map_or(None, |_| Some(user.user_uuid.clone()))
+    }
+
+    fn delete_user(&mut self, user_uuid: String) {
+        // TODO: Remove user from `username_to_user` and `uuid_to_user`.
+        if let Some(user) = self.uuid_to_user.get(&user_uuid) {
+            self.username_to_user.remove(&user.username);
+            self.uuid_to_user.remove(&user_uuid);
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn should_create_user() {
+        let mut user_service = UsersImpl::default();
+        user_service
+            .create_user("username".to_owned(), "password".to_owned())
+            .expect("should create user");
+
+        assert_eq!(user_service.uuid_to_user.len(), 1);
+        assert_eq!(user_service.username_to_user.len(), 1);
+    }
+
+    #[test]
+    fn should_fail_creating_user_with_existing_username() {
+        let mut user_service = UsersImpl::default();
+        user_service
+            .create_user("username".to_owned(), "password".to_owned())
+            .expect("should create user");
+
+        let result = user_service.create_user("username".to_owned(), "password".to_owned());
+
+        assert!(result.is_err());
+    }
+
+    #[test]
+    fn should_retrieve_user_uuid() {
+        let mut user_service = UsersImpl::default();
+        user_service
+            .create_user("username".to_owned(), "password".to_owned())
+            .expect("should create user");
+
+        assert!(user_service
+            .get_user_uuid("username".to_owned(), "password".to_owned())
+            .is_some());
+    }
+
+    #[test]
+    fn should_fail_to_retrieve_user_uuid_with_incorrect_password() {
+        let mut user_service = UsersImpl::default();
+        user_service
+            .create_user("username".to_owned(), "password".to_owned())
+            .expect("should create user");
+
+        assert!(user_service
+            .get_user_uuid("username".to_owned(), "incorrect password".to_owned())
+            .is_none());
+    }
+
+    #[test]
+    fn should_delete_user() {
+        let mut user_service = UsersImpl::default();
+        user_service
+            .create_user("username".to_owned(), "password".to_owned())
+            .expect("should create user");
+
+        let user_uuid = user_service
+            .get_user_uuid("username".to_owned(), "password".to_owned())
+            .unwrap();
+
+        user_service.delete_user(user_uuid);
+
+        assert_eq!(user_service.uuid_to_user.len(), 0);
+        assert_eq!(user_service.username_to_user.len(), 0);
+    }
+}
diff --git a/src/health-check-service/main.rs b/src/health-check-service/main.rs
index b1ee2c3..8d26482 100644
--- a/src/health-check-service/main.rs
+++ b/src/health-check-service/main.rs
@@ -1,3 +1,70 @@
-fn main() {
-    println!("health check service");
+use std::env;
+
+use authentication::auth_client::AuthClient;
+use authentication::{SignInRequest, SignOutRequest, SignUpRequest};
+use tokio::time::{sleep, Duration};
+use tonic::{Request, Response};
+use uuid::Uuid;
+
+use crate::authentication::{SignInResponse, SignOutResponse, SignUpResponse, StatusCode};
+
+pub mod authentication {
+    tonic::include_proto!("authentication");
+}
+
+#[tokio::main]
+async fn main() -> Result<(), Box<dyn std::error::Error>> {
+    // AUTH_SERVICE_HOST_NAME will be set to 'auth' when running the health check service in Docker
+    // ::0 is required for Docker to work: https://stackoverflow.com/questions/59179831/docker-app-server-ip-address-127-0-0-1-difference-of-0-0-0-0-ip
+    let auth_hostname = env::var("AUTH_SERVICE_HOST_NAME").unwrap_or("[::0]".to_owned());
+
+    // Establish connection when auth service
+    let mut client = AuthClient::connect(format!("http://{}:50051", auth_hostname)).await?;
+
+    loop {
+        let username: String = Uuid::new_v4().to_string(); // Create random username using new_v4()
+        let password: String = Uuid::new_v4().to_string(); // Create random password using new_v4()
+
+        let request: Request<SignUpRequest> = Request::new(SignUpRequest {
+            username: username.clone(),
+            password: password.clone(),
+        }); // Create a new `SignUpRequest`.
+
+        let response: Response<SignUpResponse> = client.sign_up(request).await?; // Make a sign up request. Propagate any errors.
+
+        // Log the response
+        println!(
+            "SIGN UP RESPONSE STATUS: {:?}",
+            StatusCode::from_i32(response.into_inner().status_code)
+        );
+
+        // ---------------------------------------------
+
+        let request: Request<SignInRequest> = Request::new(SignInRequest { username, password }); // Create a new `SignInRequest`.
+
+        // Make a sign in request. Propagate any errors. Convert Response<SignInResponse> into SignInResponse.
+        let response: SignInResponse = client.sign_in(request).await?.into_inner();
+
+        println!(
+            "SIGN IN RESPONSE STATUS: {:?}",
+            response.status_code // Log response status_code
+        );
+
+        // ---------------------------------------------
+
+        let request: Request<SignOutRequest> = Request::new(SignOutRequest {
+            session_token: response.session_token,
+        }); // Create a new `SignOutRequest`.
+
+        let response: Response<SignOutResponse> = client.sign_out(request).await?; // Make a sign out request. Propagate any errors.
+
+        println!(
+            "SIGN OUT RESPONSE STATUS: {:?}",
+            response.into_inner().status_code // Log response status_code
+        );
+
+        println!("--------------------------------------",);
+
+        sleep(Duration::from_secs(3)).await;
+    }
 }