The cohomology groups of Lie superalgebras and, more generally, of ε Lie algebras, are introduced and investigated. The main emphasis is on the case where the module of coefficients is non-trivial. Two general propositions are proved, which help to calculate the cohomology groups. Several examples are included to show the peculiarities of the super case. For L = sl(1|2), the cohomology groups H 1 (L, V ) and H 2 (L, V ), with V a finite-dimensional simple graded L-module, are determined, and the result is used to show that H 2 (L, U (L)) (with U (L) the enveloping algebra of L) is trivial. This implies that the superalgebra U (L) does not admit of any non-trivial formal deformations (in the sense of Gerstenhaber). Garland's theory of universal central extensions of Lie algebras is generalized to the case of ε Lie algebras.q-alg/9701037