Bousfield localization is a sophisticated version of the general idea of localization. We can localize a category by formally inverting certain morphisms: for example, when forming the homotopy category of a model category, where we invert morphisms called ‘weak equivalences’. But Bousfeld localization is a subtler process. In the case of a model category, Bousfield localization allows us to make more morphisms count as weak equivalences. There is a related notion of Bousfield localization for triangulated categories.