The haematological malignancies comprise the acute and chronic leukaemias, myelodysplastic syndromes (MDS), chronic myeloproliferative disorders, Hodgkin's disease and non-Hodgkin's lymphomas and multiple myeloma. This chapter is devoted to the rare but problematic situation in which pregnancy occurs during therapy or, more commonly, the haematological disease is diagnosed as a result of simple blood investigations during an established pregnancy. The acute leukaemias reflect a situation of maturation arrest in blood cell development resulting in proliferation and accumulation of immature precursors of the myeloid or lymphoid lineages. Chronic myeloid leukaemia (CML) typically presents in the chronic phase but terminates in blast crisis, a condition very similar to, but more resistant to therapy than acute leukaemia. Appropriate management will depend on due consideration of the underlying disease, the disease status, its treatment, and the immediacy of the requirement for that treatment.