In this paper we present a combinatorial machinery, consisting of a graph tower Ð Ý Γ and vector towers Ð Ý v on Ð Ý Γ , which allows us to efficiently describe all invariant measures µ " µ Ð Ý v on any given shift space over a finite alphabet.The new technology admits a number of direct applications, in particular concerning invariant measures on non-primitive substitution subshifts, minimal subshifts with many ergodic measures, or an efficient calculation of the measure of a given cylinder. It also applies to currents on a free group F N , and in particular the set of projectively fixed currents under the action of a (possibly reducible) endomorphism ϕ : F N Ñ F N is determined, when ϕ is represented by a train track map.