Invariant manifolds provide the geometric structures for
describing and understanding dynamics of nonlinear systems. The
theory of invariant manifolds for both finite- and infinite-dimensional
autonomous deterministic systems
and for stochastic ordinary differential equations
is relatively mature. In this paper,
we present a unified theory of invariant manifolds for infinite-dimensional
random dynamical systems generated by stochastic partial differential equations.
We first introduce a random graph transform and a fixed point
theorem for nonautonomous systems. Then we show the existence of
generalized fixed points which give the desired invariant
manifolds.