This paper describes a procedure for unification in extensional higher order logics. The procedure is due to Gerard Huet and was described in <Hu75>. In this paper we consider unification in extensional theories, which is a significantly simpler problem and is not described in detail in Huet's paper. In addition, the description here focuses on computation of substitutions rather than deciding unifiability, as in Huet's paper. To this some explanatory and clarifying material has been added.