Recent studies of moonlighting functions and catalytic promiscuity provide insights into the structural and mechanistic bases of these phenomena. Moonlighting proteins that are highlighted include gephyrin, the Neurospora crassa tyrosyl tRNA synthetase, phosphoglucose isomerase, and cytochrome c. New insights into catalytic promiscuity are provided by studies of aminoglycoside kinase (3') type IIIa, tetrachlorohydroquinone dehalogenase, and aldolase antibody 38C2.