This paper presents new bounds for heterogeneous plates which are similar to the well-known Hashin-Shtrikman bounds, but take into account plate boundary conditions. The Hashin-Shtrikman variational principle is used with a self-adjoint Greeen-operator with traction-free boundary conditions proposed by the authors. This variational formulation enables to derive lower and upper bounds for the effective in-plane and out-of-plane elastic properties of the plate. Two applications of the general theory are considered: first, in-plane invariant polarization fields are used to recover the ”first-order” bounds proposed by Kolpakov(1999) for general heterogeneous plates; next, ”second-order bounds” for n-phase plates whose constituents are statistically homogeneous in the in-plane directions are obtained. The results related to a two-phase material made of elastic isotropic materials are shown. The ”second-order” bounds for the plate elastic properties are compared with the plate properties of homogeneous plates made of materials having an elasticity tensor computed from ”second-order” Hashin-Shtrikman bounds in an infinite domain.