The graft‐versus‐leukaemia (GVL) effect is a central component of the stem cell allograft’s ability to cure haematological malignancies. The GVL effect is mediated by donor‐derived natural killer cells and T lymphocytes, which have distinct mechanisms of recognizing and targeting the recipient’s malignant cells. After transplantation the cytokine milieu is favourable to the early establishment of a GVL effect, but the need to prevent graft‐versus‐host disease limits the full potential of this process. Clinical studies have identified some critical components of the transplant preparation, donor selection, stem cell source (peripheral blood versus bone marrow) and post‐transplant management that can be manipulated to optimize the GVL effect. However, further developments focusing on the selective depletion of unwanted alloreactivity with preservation of GVL effects, and the use of vaccines or the adoptive transfer of leukaemia‐specific lymphocytes, will be required to enhance the GVL effect to reliably eradicate more resistant leukaemias.