The evolutionarily conserved WNT-signalling pathway has pivotal roles during the development of many organ systems, and dysregulated WNT signalling is a key factor in the initiation of various tumours. Recent studies have implicated a role for WNT signal transduction at several stages of lymphocyte development and in the self-renewal of haematopoietic stem cells. Here, we outline new insights into the WNT-signalling pathway, review its role in the self-renewal of haematopoietic stem cells and in the development of T and B cells, and discuss controversies and future developments with regard to WNT signalling in the thymus.