Computer modelling has become an integral tool in the analysis and understanding of the reaction networks that underlie cellular processes. Programs such as the Systems BiologyWorkbench (SBW) and COPASI allow us to simulate the behaviour of these reaction networks each has its advantages and limitations. The need to adapt modelling software to our specific needs prompted the development of PySCeS, the Python Simulator for Cellular Systems, which we present here. PySCeS is an extremely exible, user-extensible, open-source modelling tool. Development started in 2000, with the first public release following in 2004. PySCeS is written in Python and makes extensive use of the highly successful IPython, NumPy, SciPy, Matplotlib stack. It has been developed to run on both Microsoft Windows and Linux (with a Mac OS X port possible in principle) and is currently released under the GNU GPL licence.