Notch signalling participates in the development of multicellular organisms by maintaining the self-renewal potential of some tissues and inducing the differentiation of others. Involvement of Notch in cancer was first highlighted in human T-cell leukaemia, fuelling the notion that aberrant Notch signalling promotes tumorigenesis. However, there is mounting evidence that Notch signalling is not exclusively oncogenic. It can instead function as a tumour suppressor.