Drug discovery targeting novel mechanisms has become extremely expensive and risky. The annual first-in-class drug approvals have not been satisfactory in the past decade (two to six per year) despite an increased R&D budget. Follow-on programs targeting proven mechanisms are less risky and costly but can produce drugs with meaningful differentiations and thus can play an important supporting role. This article will discuss the medicinal chemistry strategies that have been utilized by the pharmaceutical industry to exploit validated therapeutic targets.