A spectral formulation for the analysis of arrays of thin conductors arranged in a skew three-dimensional lattice and lying in one of the lattice planes is presented. Two cases are considered, i.e. homogeneous and plane stratified dielectric medium. In the first case, the Fourier transformation is three-dimensional; in the second one, the spectral approach is Limited to the plane of stratification and the transmission line technique is used for the analysis in the longitudinal direction. The eigenmodes of the structure (Bloch waves) are studied in detail and some significant examples of dispersion curves are presented. The concepts of passband and stopband of Bloch waves arc exploited to define a synthesis procedure for high performance multiple grid frequency selective surfaces, which are considered as three-dimensional periodic structures truncated in the direction of a lattice basis vector.