We establish existence and multiplicity of periodic solutions to the infinite dimensional Hamiltonian system GRAPHICS where Omega subset of R-N is a bounded domain or Omega = RN. When Omega is bounded, we treat the situations where H(t, x, z) is, with respect to z = (u, v), sub- or superquadratic, or concave and convex, and discuss also the convergence to homoclinics of sequences of subharmonic orbits. If Omega = R-N, we handle the case of superquadratic nonlinearities.