Designing application and middleware software to run in concurrent and networked environments is a significant challenge to software developers. The patterns catalogued in this second volume of Pattern-Oriented Software Architectures (POSA) form the basis of a pattern language that addresses issues associated with concurrency and networking. The book presents 17 interrelated patterns ranging from idioms through architectural designs. They cover core elements of building concurrent and network systems: service access and configuration, event handling, synchronization, and concurrency. All patterns present extensive examples and known uses in multiple programming languages, including C++, C, and Java. The book can be used to tackle specific software development problems or read from cover to cover to provide a fundamental understanding of the best practices for constructing concurrent and networked applications and middleware.