The gene encoding lipin 1 was identified with a positional cloning approach that localized the causative mutation in fatty liver dystrophic (fld) mice, a mouse model of lipodystrophy. The fld mouse lacks normal adipose tissue in the body, and displays metabolic dysregulation such as obesity, insulin resistance, and hypertriglyceridemia. Lipin 1 is abundantly expressed in key metabolic tissues, including adipose tissue, skeletal muscle, and liver. In the cytosol, lipin 1 acts as an Mg²⁺-dependent phosphatidate phosphatase type-1 (PAP1), catalyzing a key step in the synthesis of glycerolipids. In the nucleus, lipin 1 acts as a transcriptional coactivator through its direct interaction with peroxisome proliferator-activated receptor (PPAR) γ coactivator-1α (PGC-1α) and PPARα. Through two distinct functions in the nucleus and cytosol, lipin 1 modulates lipid metabolism and glucose homeostasis. Here we will discuss recent developments in our understanding of the role of lipin 1 in lipid metabolism.