Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. Parallel tempering, also known as replica exchange, is a simulation method aimed at improving the dynamic properties of Monte Carlo method simulations of physical systems, and of Markov chain Monte Carlo sampling methods more generally. The replica exchange method was originated by Swendsen, then extended by Geyer and later developed, among others, by Giorgio Parisi. Typically an MC simulation using a Metropolis-Hastings update consists of a single stochastic process that evaluates the energy of the system and accepts/rejects updates based on the temperature T. At high temperatures updates that change the energy of the system are comparatively more probable. When the system is highly correlated, updates are rejected and the simulation is said to suffer from critical slowing down.