A review of the material technology for microwave integrated circuits (MICs) is presented. The types of microwave circuit media that have been used are described and classified as a function of the amount of size reduction or integration which corresponds to the effective dielectric constant of the media. The materials used for substrates, conductors, dielectrics, and resistors are considered in terms of the requirements for microwave circuits. The fabrication of multilayered thin-film circuits and the various thin-film combinations that have been used in MICs are discussed. The various loss contributions for microstrip circuits produced by thin- and thick-film technology and substrate material are compared with each other as a function of frequency. It is concluded that microwave circuits operating at frequencies 2 GHz and above require thin-films on pure smooth substrates.