This issue attempts to give a 'state of the art' overview of the AP-1 transcription factor family, a fundamental class of transcriptional regulators. The AP-1 family consists of several bZIP (basic region leucine zipper) domain proteins, the Jun, the Fos, and the ATF subfamilies, which all have to dimerize before they can bind to their DNA target sites. AP-1 has been a paradigm for transcription factors that regulate many aspects of cell physiology in response to environmental changes such as stress, radiation, or to growth factor signals thereby acting like an environmental biosensor. Although we have come a long way from discovering its major components, the heterodimer between c-Fos and c-Jun, it is daunting to realize that we still lack a detailed molecular knowledge of how these factors interact with DNA to activate or repress genes in the nucleus. It is also not clear how the response of AP-1 to growth factor signaling from the cell surface to the nucleus is interpreted at the molecular level and whether AP-1 is relevant for human disease.