In Erlang, process communication and message passing are fundamental concepts for achieving concurrency and building distributed systems. Erlang processes communicate with each other by exchanging messages using asynchronous message passing. Here's an overview of how Erlang handles process communication and message passing:

1. Process Creation : Erlang allows the creation of lightweight processes using the `spawn/1` or `spawn/3` functions. Each process is identified by a unique process identifier (PID).

2. Sending Messages : To send a message from one process to another, you use the `!` operator. For example, `PID ! Message` sends `Message` to the process identified by `PID`. The sender does not wait for a response but continues its execution immediately.

3. Receiving Messages : Processes can receive messages using the `receive` construct. It allows a process to wait for specific messages and pattern match on the received messages to determine the appropriate action to take.

4. Pattern Matching : When receiving a message, Erlang uses pattern matching to match the incoming message against patterns specified in the `receive` block. Messages that match a pattern are processed, while non-matching messages remain in the process's mailbox until a matching pattern is encountered. 5. Selective Receive : Erlang provides selective receive, which allows processes to prioritize specific messages over others. By specifying different patterns in the receive block, processes can selectively handle certain messages based on their content or structure.

6. Asynchronous Communication : Erlang's message passing is asynchronous. The sender does not block or wait for a response when sending a message. This non-blocking nature enables concurrent and scalable communication between processes.

7. Process Mailboxes : Erlang maintains a mailbox for each process, where incoming messages are stored until the process retrieves and processes them. The mailbox follows a first-in-first-out (FIFO) order, ensuring fairness and message ordering within a process.

8. Message Ordering : Erlang guarantees that messages sent from one process to another are received in the order they were sent. This order is maintained within a process's mailbox, ensuring predictable message processing.

9. Message Delivery : Erlang's message passing is reliable, meaning that messages are delivered even if the receiver process is temporarily busy or unavailable. Messages remain in the receiver's mailbox until the process is ready to handle them.

10. System Monitoring : Erlang provides mechanisms for monitoring the state of processes, such as detecting process termination or failures. Monitoring allows processes to react to changes in the state of other processes and enables fault-tolerant behaviors.