From stealth materials to dielectric substrates, microwave food products to biofuels, accurate characterization of their electromagnetic properties at microwave and mm-wave frequencies provide engineers with critical information needed for material and circuit design, modeling, research, manufacturing and quality control. In this paper we will overview the classes of methods employed to measure the dielectric properties of solids and liquids and will discuss the criteria one should consider when selecting a measurement technique. The focus will be on those techniques useful for measuring the relative permittivity and loss tangent of dielectric materials, both liquid and solid, over a frequency range that covers 100 MHz to 1.1 THz.