A number of III-V compound semiconductors have higher electron mobilities and peak electron velocities than silicon. This makes these materials attractive for use in high-frequency applications. However, the use of these materials was delayed for a number of years by the fact that most of these compounds such as GaAs lack a stable native oxide and display a large interface trap density at the junction with other materials. These problems were overcome in the early 1960s by the development of the MESFET (metal semiconductor FET) where the control electrode was formed by a metal Schottky contact directly onto the GaAs surface. Early devices were fabricated in epitaxial conducting layers deposited, by for example vapor phase epitaxy, onto GaAs substrates. As the quality of these substrates improved, the conducting layers were formed by implantation of silicon ions directly into the substrate surface. The requirement for devices working at even higher frequencies has necessitated the move from implanted MESFETs to more complicated multi-layer heterostructures, for devices such as HEMTs (high electron mobility transistors) and HBTs (heterojunction bipolar transistors), which can only be produced using epitaxial techniques. In this article the technique of ion-implantation and molecular beam epitaxy are described together with a description of HEMTs and HBTs.