In many species, the lactating mammary gland is one of the most lipogenic organs of the body. The majority of the lipid produced during lactation is secreted into milk by a novel process of membrane envelopment of cytoplasmic lipid droplets (CLDs). Adipophilin (ADRP/ADPH/PLIN2), a member of the perilipin (PAT) family of lipid droplet proteins, is hypothesized to play a pivotal role in both formation and secretion of milk lipids. Production of milk lipids is the only known example of CLD secretion, and the only process in which PAT family members undergo secretion. This review discusses emerging data about the structural and functional properties of adipophilin that determine its physiological actions and mediate its effects on milk lipid formation and secretion.