Mitogen-activated protein (MAP) kinases are proline-directed serine/threonine kinases that are activated by dual phosphorylation on threonine and tyrosine residues in response to a wide array of extracellular stimuli. Three distinct groups of MAP kinases have been identified in mammalian cells [extracellular-regulated kinase (ERK), c-Jun N-terminal kinase (JNK), and p38]. These MAP kinases are mediators of signal transduction from the cell surface to the nucleus. One nuclear target of these MAP kinase signaling pathways is the transcription factor AP-1. MAP kinases regulate AP-1 transcriptional activity by multiple mechanisms. Here we review recent progress towards understanding AP-1 regulation by the ERK, JNK, and p38 MAP kinase signal transduction pathways.