Equivalence checking is a formal verification approach that consists in proving that two programs or models are related modulo some equivalence relation, or that one is included in the other modulo some preorder relation. In the case of concurrent systems, which are often represented using labelled transition systems, the relations used for equivalence checking are bisimulations and their simulation preorders. In the case of probabilistic or stochastic systems, which are usually represented using Markov chains, the relations used for equivalence checking are lumpability, probabilistic and stochastic equivalences, and their associated preorders. The present article provides a synthetic overview of 40 years of research in the design of algorithms and software tools for equivalence checking.