Abstract. This work generalizes the short resolution given in Proc. Amer. Math. Soc. 131, 4, (2003), 1081-1091, to any affine semigroup. Moreover, a characterization of Apéry sets is given. This characterization lets compute Apéry sets of affine semigroups and the Frobenius number of a numerical semigroup in a simple way. We also exhibit a new characterization of the Cohen-Macaulay property for simplicial affine semigroups.