In this thesis the basic theory of shielding effectiveness is described in Chapter 1, where only approximate solutions are presented to compute the shielding effectiveness of metallic shields. The purpose of this chapter is to introduce the basic concepts of the electromagnetic waves shielding that will be recalled in Chapter 2, where all the actually used methods to measure the shielding effectiveness of materials are described. This chapter is a kind of selection guide that allows the reader to choose the best method for measuring the shielding effectiveness of its material. Only two of these methods are standardized, because the research of new materials able to shield the electromagnetic waves is relatively new.