This paper describes the fundamental concepts of semi-active control, and the development and testing of a semi-active suspension on an M551 tank. The single degree-of-freedom (SDOF) model of a vehicle suspension is used in explaining the basic concepts. Background information is presented on conventional, fully active, and optimal suspension systems. One goal of this paper is to give the reader an intuitive “feel” for semi-active control. Primary vehicle suspension systems are a natural application for semi-active control, but the concepts apply equally well to general vibration isolation problems.