This book describes superconductor/two-dimensional-electron-gas (2DEG) structures with the aim of providing a better understanding of their transport properties. The two-dimensional electron gas used for the structures described here is buried either in a III-V semiconductor heterostructure or in an inversion layer at the surface. First, the properties of a single superconductor/2DEG structure are discussed on the basis of Andreev reflection and the proximity effect. Then, the properties of superconductor/2DEG Josephson junctions are treated. The last part of the book discusses the control of junctions using gate electrodes or injection currents, and the physical effects observed in these junctions.