The past several years have seen an increasing interest in the peroxisome proliferator-activated receptors (PPARs). These transcriptional factors belong to the superfamily of the steroid/thyroid/retinoid receptors. They are activated by fatty acids or their metabolites as well as by different xenobiotic peroxisome proliferators. These receptors are expressed in both the embryo and the adult organism. They have been implicated in cell proliferation, differentiation and apoptosis. In this review, we will attempt to point out some of the more salient features of this expression pattern during development and the different steps of cell life. The current understanding of how PPARs are involved in some human diseases will also be described.