Although there has been strong demand for low -resistance, highly reliable Ohmic contacts for compound semiconductors to realize high-frequency transistors, high-power devices, and light-emitting (LED) and laser diodes (LD), development of the Ohmic contact technologies has been made on a trial-and-error basis. The primary reason is lack of fundamental data to design ideal metal/semiconductor interfaces due to complexity of elements involved at the interfacial reaction. In this article, we review recent systematic studies carried out for Ohmic contact materials to n-GaAs and then address critical issues to apply the methodology established in n-GaAs to develop low-resistance Ohmic contacts for p-ZnSe and p-GaN, which are desperate for blue-green LED and LD.