Equilibrium states are invariant measures that balance their potential energy with their entropy. This approach to selecting invariant measures mirrors ideas from statistical mechanics. The thermodynamic pressure is the supremum, taken over all invariant measures, of the sum of potential energy and entropy. Equilibrium states of Holder potentials of uniformly hyperbolic systems are fairly well understood; this theory goes back to Sinai, Ruelle and Bowen in the early 1970s. Since around 2000, the field has regained interest as the knowledge of nonuniformly hyperbolic systems increased with the advent of new tools, such as (Young) towers and Markov extensions.