Book excerpt: 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 over a decade ago. Over the years this project has accumulated an extensive body of sample code and extensions. Currently SIMPL libraries are supported for C/C++, Python, JAVA and Tcl programming languages. A sample Sudoku puzzle solver project illustrates all these in a single application.