The evolution of weakly nonlinear modulated disturbances in marginally unstable systems has been the subject of intensive study. In a wide range of problems it has been proposed that such disturbances are described by the (complex) Ginzburg–Landau equation, or in the case of two–dimensional modulations in certain systems, by the Davey–Stewartson equations with complex coefficients. In this paper, we re–examine the evolution of an initially linear localized disturbance into the weakly nonlinear regime. We show that for one–dimensional modulations, the weakly nonlinear evolution is governed, initially at least, by the Landau equation, rather than the Ginzburg–Landau equation. For two–dimensional modulations, the evolution is governed by a reduced form of the Davey–Stewartson equations. We apply our revised theory to three–dimensional Poiseuille–Couette flow. For both one– and two–dimensional modulations we find that a localized disturbance can always grow to order–one amplitude if its initial magnitude is sufficiently small (contrary to earlier predictions).