T-cell acute lymphoblastic leukaemia (T-ALL) is induced by the transformation of T-cell progenitors and mainly occurs in children and adolescents. Although treatment outcome in patients with T-ALL has improved in recent years, patients with relapsed disease continue to have a poor prognosis. It is therefore important to understand the molecular pathways that control both the induction of transformation and the treatment of relapsed disease. In this Review, we focus on the molecular mechanisms responsible for disease induction and maintenance. We also compare the physiological progression of T-cell differentiation with T-cell transformation, highlighting the close relationship between these two processes. Finally, we discuss potential new therapies that target oncogenic pathways in T-ALL.