  We prove an abstract Birkhoff normal form theorem for Hamiltonian Partial
Differential Equations. The theorem applies to semilinear equations with
nonlinearity satisfying a property that we call of Tame Modulus. Such a
property is related to the classical tame inequality by Moser. In the
nonresonant case we deduce that any small amplitude solution remains very close
to a torus for very long times. We also develop a general scheme to apply the
abstract theory to PDEs in one space dimensions and we use it to study some
concrete equations (NLW,NLS) with different boundary conditions. An application
to a nonlinear Schr\"odinger equation on the $d$-dimensional torus is also
given. In all cases we deduce bounds on the growth of high Sobolev norms. In
particular we get lower bounds on the existence time of solutions.
