Designing and analyzing the Frequency Selective Surfaces (FSS) is reported in this thesis. FSS structures are organized in the group of resonators structures. The arrangement allows to transmit and reflect and in some cases absorb the electromagnetic waves in a given frequency band. These structures are widely used in radoms, reducing radar cross-section (RCS), etc. In this work it is concentrated to design a FSSs by the incorporation PIN diodes to make three structures tunable. The purpose of using diodes is to overcome the difficulty of FSS structures in terms of reflection/transmission characteristic which is dependent on the incidence angle. Implementing PIN diodes is one way among other solutions, which have been attracted the designers` attention.