COPASI is a package for modeling and simulation of biochemical networks, popular in the field of systems biology. COPASI is a stand-alone program that simulates models of biochemical networks using ODE solvers or Gillespie's stochastic simulation algorithm. COPASI is compatible with models in SBML format. It also performs several analyses, such as steady state, stability, parameter sensitivity, elementary modes, Lyapunov exponents, optimization, and parameter estimation. Data can be visualized in plots, histograms and animations of network diagrams. COPASI's GUI is based on QT, but a command line version is also included that allows for processing computations in batch mode. COPASI is a collaboration between research groups at the Virginia Bioinformatics Institute, University of Heidelberg, University of Manchester and previously in the EML-Research.