Peroxisome proliferator-activated receptors (PPARs) are nuclear receptors and have been initially described as molecular targets for compounds which induce peroxisome proliferation. The interest of researchers for PPARs increased dramatically when these receptors were shown to be directly activated by a number of medically relevant compounds. These compounds include: the fibrate class of hypolidemic drugs, the thiazolidinediones, which are insulin sensitizers used as orally active antidiabetic agents, certain non-steroidal anti-inflammatory drugs (NSAIDs), and naturally occurring fatty acid-derived molecules. Rapidly, it was demonstrated that PPARs are key regulators of lipid homeostasis and provide a molecular link between nutrition and gene regulation. Recently, detailed studies of PPAR expression profiles in different tissues pointed to the roles these receptors play in inflammation control and cell proliferation. In this review we will focus on the new insights gained into these two areas and we will also discuss our current knowledge of the regulation of PPAR transcriptional activity by cofactors.