Abstract. For systems of partial differential equations (PDEs) with locally cubic nonlinearities, which are perturbed by additive noise, we describe the essential dynamics for small solutions. If the system is near a change of stability, then a natural separation of time-scales occurs and the amplitudes of the dominant modes are given on a long time-scale by a stochastic ordinary differential equation. We consider applications to dynamic pitchfork bifurcation, pattern formation below the threshold of stability, and transient dynamics of stochastic PDEs near this deterministic bifurcations.Key words. amplitude equation, pattern formation, SPDE, slow modes, separation of time-scales, approximate center-manifold, bifurcation, multiple scale analysis AMS subject classifications. 60H15, 60H10, 37L55, 37L65DOI. 10.1137/S11111111034213551. Introduction. Amplitude equations are well known in the physics literature (see, e.g., [H83] or [W97]). They usually describe some order parameter for the system, which evolves on a much slower time-scale. This separation of time-scales occurs, for example, very naturally in a small neighborhood of bifurcations, where a change of stability occurs.Amplitude equations can be used either for spatially extended systems, where they are stochastic partial differential equations (SPDEs), or for systems on bounded domains, where they are given as stochastic ordinary differential equations (SODEs). This paper will focus on the second case, where an SODE describes the dynamics of the amplitudes of dominant modes evolving on some slow time-scale. On the other hand, all nondominant modes evolve rapidly on a fast time-scale, but they stay much smaller than the dominant ones. The modes in our context are given by the Fourier series expansion with respect to the eigenfunctions of the corresponding linearized operator. For deterministic systems the theory is rigorously understood even for spatially extended systems (see, e.g.,[KSM92,vH91] for the first results). However, there is a lack of results for stochastic systems. The only rigorous example is [BMS01] for a stochastic Swift-Hohenberg equation with periodic boundary conditions on a bounded interval. In this example, a complexvalued SODE was derived describing the amplitude of the dominant mode in a standard complex Fourier series on a very long time-scale.Our main theorems will extend the results of [BMS01] to a large class of SPDEs and systems of SPDEs. Moreover, our applications will demonstrate the power of this approach, when describing transient dynamics of stochastic equations.