At its most fundamental SIMPL is a set of library functions which allow the passing of encapsulated messages between cooperating processes. These processes may be local to one host computer or spread over a network. On another level SIMPL is an active open source project which began in the 1990's. Over the years this project has accumulated an extensive body of sample code and extensions. Recently a kernel module variation called SRY has been added. This book is the third edition which updates and extends the body of work.