CosmoMC is a Fortran 2008 Markov-Chain Monte-Carlo (MCMC) engine for exploring cosmological parameter space, together with Fortran and python code for analysing Monte-Carlo samples and importance sampling (plus a suite of scripts for building grids of runs, plotting and presenting results). The code does brute force (but accurate) theoretical matter power spectrum and Cl calculations with CAMB. See the original paper for an introduction and descriptions, and up-to-date sampling algorithm details. It can also be compiled as a generic sampler without using any cosmology codes.