Fas is a transmembrane receptor that can induce apoptosis after cross-linking with either agonistic antibodies or with Fas ligand (FasL). Although originally described as an important regulator of peripheral immune homeostasis, accumulating evidence suggests that the Fas/FasL system plays an important role in tumour development. In addition to its proapoptotic functions, accumulating evidence demonstrates that Fas can activate numerous nonapoptotic signalling pathways, and that activation of these pathways can result in increased tumourigenicity and metastasis. This review summarises the current understanding of the Fas/FasL system in tumorigenesis and discusses attempts to utilise the Fas/FasL system in the treatment of cancer.