The basic constitutive equations and equations of motion are derived for anisotropic porous piezoelectric materials by making use of variational principle. These equations are first derived for three-dimensional case and then reduced to the two-dimensional case by expanding mechanical displacement and electric potential as power series. The results are obtained in particular for monoclinic (2) porous piezoelectric materials. Thickness shear correction factors are determined for high frequency case. The thickness shear resonance frequencies are obtained numerically for a particular model of PZT.