We consider stochastic dynamical systems on R, that is, random processes defined by X x n = Ψn(X x n−1 ), X x 0 = x, where Ψn are i.i.d. random continuous transformations of some unbounded closed subset of R. We assume here that Ψn behaves asymptotically like Anx, for some random positive number An [the main example is the affine stochastic recursion Ψn(x) = Anx + Bn]. Our aim is to describe invariant Radon measures of the process X x n in the critical case, when E log A1 = 0. We prove that those measures behave at infinity like dx x . We study also the problem of uniqueness of the invariant measure. We improve previous results known for the affine recursions and generalize them to a larger class of stochastic dynamical systems which include, for instance, reflected random walks, stochastic dynamical systems on the unit interval [0, 1], additive Markov processes and a variant of the Galton-Watson process.