Frequency selective surfaces (FSS) have numerous applications in microwave and optical systems. In this work, instead of traditional dielectric substrates, ferrite substrates are used. By using ferrite materials, one can change the spectral properties of these structures without physically altering them. The transmission matrices for the ferrite substrate and the air, above and below the ferrite substrate, are derived here. By combining these transmission matrices along with the boundary conditions, the spectral domain Green's function is obtained. This process can be carried out for both the in-plane and perpendicular bias of the ferrite. Several results are presented to show the tunability of frequency selective surfaces with ferrite substrates as a function of the applied dc bias. Some special treatment is used in the transmission matrix to avoid its singularity for some evanescent modes.