BRAF is a serine threonine kinase that forms a component of the Ras Raf Mek Erk signalling pathway. Somatic point mutations in BRAF were discovered in 2002 in the early phases of a systematic screen for somatic mutations in cancer. Since then a remarkable explosion of research activity has revealed that BRAF has a distinctive spectrum of mutations that are found in several classes of cancer including melanoma, papillary thyroid cancer, and colorectal cancer and that mutations are often present at early stages of neoplastic progression. Indeed, BRAF mutations have become markers of subtypes of some cancers and are associated with particular pathways to oncogenesis. BRAF mutations tend to be found in the classes of cancer that were previously known to carry RAS gene mutations, but samples with BRAF mutations do not usually carry RAS mutations. Germline mutations of BRAF have recently been reported in rare syndromes with developmental abnormalities. BRAF mutations usually result in activation of the kinase activity and direct phosphorylation of MEK, but some do so indirectly through CRAF, another member of the RAF family. Building on previous successes of inhibitors directed at mutated and activated kinases, BRAF and the pathways it regulates have become targets for the development of small molecule drugs which are currently being introduced into clinical trials.