Flexible software for the Bayesian analysis of complex statistical models using Markov chain Monte Carlo (MCMC) methods.The BUGS (Bayesian inference Using Gibbs Sampling) project is concerned with flexible software for the Bayesian analysis of complex statistical models using Markov chain Monte Carlo (MCMC) methods. The project began in 1989 in the MRC Biostatistics Unit, Cambridge, and led initially to the `Classic’ BUGS program, and then onto the WinBUGS software developed jointly with the Imperial College School of Medicine at St Mary’s, London. Developments were later focused on OpenBUGS, an open source equivalent of WinBUGS.