The objective of a rendering algorithm is to compute a photograph of a simulated reality, which entails finding all the paths along which light can flow-from a set of light sources to the camera. The purpose of this article is to present a high-level overview of the underlying physics and analyze how this leads to a high-dimensional integration problem that is typically handled using Monte Carlo methods. Following this, we survey recent work on path space Markov Chain Monte Carlo (MCMC) methods that compute the resulting integrals using proposal distributions defined on sets of light paths.