SOSTOOLS is a free MATLAB toolbox for formulating and solving sum of squares (SOS) optimization programs. It uses a simple notation and a flexible and intuitive high-level user interface to specify the SOS programs. Currently these are solved using SeDuMi, a well-known semidefinite programming solver, while SOSTOOLS handles internally all the necessary reformulations and data conversion. The SOS convex programming ideas used in SOSTOOLS are extremely powerful, easy to understand, and ideally suited for many diverse applications ranging from global optimization and Lyapunov function synthesis, to geometric theorem proving and quantum mechanics. The toolbox can be downloaded from either http://www.cds.caltech.edu/sostools or http://www.aut.ee.ethz.ch/~parrilo/sostools