A high performance and portable simulation engine for systems and synthetic biology. I can run on many platforms including Windows, Mac OS, and Linux. libRoadRunner is major rewrite of the original C# roadRunner developed by Frank Bergmann and Herbert Sauro. The same original functionality however remains, including the original C API, the structural analysis code, sensitivity and steady state analyses, but with significant improvements to performance, back-end design, better event handling, new C++ API and stochastic simulation support.