Serotonin N-acetyltransferase (arylalkylamine N-acetyltransferase, AANAT) catalyzes the rate-limiting step in the biosynthesis of the circadian hormone melatonin from serotonin. Although melatonin was identified 40 years ago, relatively little is known about its (patho)physiological roles, and a solid scientific foundation is still lacking for most therapeutic applications currently claimed for melatonin. The development of potent, specific, and cell permeable inhibitors for AANAT should constitute an important strategy to address these issues. These inhibitors are also potential therapeutics for various sleep / mood disorders. This review will focus on the efforts toward developing in vitro and in vivo AANAT inhibitors, including basic mechanistic studies on AANAT, which have played an important role in design.