The prevalence of obesity in the western world has focused attention on factors that influence triglyceride biosynthesis, storage, and utilization. Members of the lipin protein family have a newly discovered enzymatic role in triglyceride and phospholipid biosynthesis as a phosphatidate phosphatase, and also act as an inducible transcriptional coactivator in conjunction with peroxisome proliferator-activated receptor gamma (PPAR gamma) coactivator-1 alpha and PPAR alpha. Through these activities, the founding member of the family, lipin-1, influences lipid metabolism and glucose homeostasis in diverse tissues including adipose tissue, skeletal muscle, and liver. The physiological roles of lipin-2 and lipin-3 are less well defined, but are likely to carry out similar functions in glycerolipid biosynthesis and gene expression in a distinct tissue distribution.