The Synthesis Toolkit (STK) is an open sourceAPI for real timeaudio synthesis with an emphasis on classes to facilitate the development of physical modelling synthesizers. It is written in C++ and is written and maintained by Perry Cook at Princeton University and Gary Scavone at McGill University. It contains both low-level synthesis and signal processing classes (oscillators, filters, etc.) and higher-level instrument classes which contain examples of most of the currently available physical modelling algorithms in use today. STK is free software, but a number of its classes, particularly some physical modelling algorithms, are covered by patents held by Stanford University and Yamaha.