Material manufacturers are developing new, high-performance substrate materials in order to meet the specifications of new applications that operate over wider and wider frequencies. In order for engineers to select the most appropriate material to incorporate into microwave devices, the electrical properties of the substrate, namely the relative permittivity and loss tangent, must be accurately measured, sometimes as a function of frequency, temperature and even humidity. However, there are hundreds of measurement methods published in the literature and selecting the proper technique can be daunting. In this lecture, we overview both transmission-line and resonant techniques for measuring the relative permittivity and loss tangent of dielectric substrates such as printed-circuit boards, printed-wiring boards and ceramic substrates over the frequency range of 1 to 100 GHz. In the area of transmission-line measurements, we outline how to perform broadband on-wafer material measurements using planar structures such as coplanar waveguides and microstrip transmission lines. Next, we overview three resonant measurement methods: the split-cylinder, split-post and Fabry-Perot resonators. These three measurement techniques are increasingly being used by industry to accurately characterize substrate materials at microwave and millimeter-wave frequencies and represent the state of the art in nondestructive materials testing. The lecture concludes with a discussion of each method’s advantages and limitations and with some final remarks about the future direction of high-frequency material measurements.