Abstract. Let L ⊂ Z n be a lattice and IL = x u −x v : u−v ∈ L be the corresponding lattice ideal in k[x1, . . . , xn], where k is a field. In this paper we describe minimal binomial generating sets of IL and their invariants. We use as a main tool a graph construction on equivalence classes of fibers of IL. As one application of the theory developed we characterize binomial complete intersection lattice ideals, a longstanding open problem in the case of non-positive lattices.