In recent years, studies in the model organism Drosophila melanogaster have contributed significant insights into the molecular and developmental biology of the AP-1 transcription factors Jun and Fos. Powerful genetic and biochemical approaches uncovered a baffling complexity and variability of the signaling connections to and from AP-1. The range of biological processes that Jun and Fos regulate in this organism is equally multi-faceted. Regulatory interactions between AP-1 and JNK, ERK, TGFbeta, Notch or other signaling systems have been implicated in the control of a multitude of embryonic and adult events, including tissue closure processes, patterning of eye, gut and wing, as well as apoptosis. Here we review the information that has been gathered on Drosophila AP-1 in signal transduction and on the developmental and cellular functions controlled by AP-1-mediated signals in the fly. Lessons learned from the studies on AP-1 in Drosophila may contribute to our general understanding, beyond species boundaries, of this fundamental class of transcriptional regulators.