To model system implementations, we define the language of Simple Processes. In this language, systems are defined in the classical style of giving a separate program for each process. Process programs use send and receive actions that need to match during execution in order to achieve a communication. We discuss how implementations of choreographies from the previous chapter can be written in terms of this language. We also formulate in our setting the key properties of parallelism, communication safety, and starvation-freedom, respectively: the capability of executing independent communications in any order; the property that processes never attempt to interact by performing incompatible actions; and the property that every running process eventually gets to act.