Extracellular-regulated protein kinase 5 (ERK5) is a mitogen-activated protein kinase (MAPK) regulated by a wide range of mitogens and cellular stresses. Since its cloning in 1995, the lack of biological tools, including antibodies and specific inhibitors, have made it one of the least studied MAPK subfamilies. The discovery that ERK5 was an important contributor to cell survival mechanisms has increased interest in this signalling pathway. The ability of inhibitors of the classical MAPK (ERK1/2) cascade to block ERK5 activation suggested that ERK5 might regulate some cellular functions originally attributed to ERK1/2. For example, ERK5 is suspected to mediate the effects of numerous oncogenes. A link between abnormal levels of ERK5 expression and cancers was established by the analysis of human tumours. Recently, the targeted deletions of the erk5 and the mek5 genes in mice have provided genetic evidence that the ERK5 cascade is a non-redundant signalling pathway essential for normal cardiovascular development. The analysis of genetically modified mice in which the erk5 gene can be specifically deleted in certain tissues is shedding light into the physiological function of the ERK5 pathway during development and pathogenesis.