We give estimates for exponential sums over finite fields in several variables. We study the case where the phase is either quadratic or more generally invariant under the action of a finite group. The bounds obtained are better than the general ones; they imply some estimates for certain sums in one variable, and for the number of solutions of the trace equation Tr(x d + vx) .---O. In an appendix we discuss the link between exponential sums and bent functions.