The peroxisome proliferator-activated receptor (PPAR) family of nuclear receptors, a set of three receptor sub-types encoded by distinct genes, function as lipid sensors to regulate a broad range of genes in many metabolically active tissues. Synthetic PPAR agonists have exhibited therapeutic benefits in treating diabetes and cardiovascular diseases. The discovery of PPAR-specific ligands has led to significant advancement in our understanding of the structure of these receptor proteins and the molecular mechanism of their ligand-dependent activation. Herein, we present both recent progress in the functional analysis of these orphan receptors and the confirmation of the PPARs as molecular targets for the development of new medicines to treat human metabolic disease.