In this chapter, I briefly present constraint reasoning. Constraint reasoning has been a subfield of artificial intelligence (AI) that is nowadays more well-known as constraint programming (CP). The change of name occurred more or less when CP has started to be widely used for solving combinatorial problems in industrial applications. This also corresponds to the moment where CP was enriched by the contributions from logic programming for the aspects related to languages and from operation research for the propagation of complex constraints. Considering the topic of this book, I will stay on a AI-oriented presentation of CP.