In developed societies, chronic diseases such as diabetes, obesity, atherosclerosis and cancer are responsible for most deaths. These ailments have complex causes involving genetic, environmental and nutritional factors. There is evidence that a group of closely related nuclear receptors, called peroxisome proliferator-activated receptors (PPARs), may be involved in these diseases. This, together with the fact that PPAR activity can be modulated by drugs such as thiazolidinediones and fibrates, has instigated a huge research effort into PPARs. Here we present the latest developments in the PPAR field, with particular emphasis on the physiological function of PPARs during various nutritional states, and the possible role of PPARs in several chronic diseases.