Scale deposited within production systems associated with high volumes of produced brine can pose serious problems to the operator both in field economics or in the total loss of production. Seven barrels of oilfield brine are produced for every barrel-equivalent of energy obtained in the United States each year and with the advent of enhanced recovery methods, this ratio can be as high as thirty or fifty to one. Many inhibitors have been in use for over fifty years, but no quantitative explanation for their effectiveness has been available. A general theory of inhibitor effectiveness and mechanisms of action are presented and two mechanisms based on diffusion rates are proposed with associated equations. Many scale inhibitors are currently on the market primarily based on phosphonates, copolymers or polymaleates. An inhibitor evaluation apparatus developed at Rice University has gained wide acceptance in the petroleum industry and will be described. The results of inhibitor evaluation tests in various brine systems will be discussed in conjunction with the theory of inhibitor effectiveness. Results are presented for CaS04 and CaC03.