The myelodysplastic syndromes (or myelodysplasias) comprise a spectrum of disorders characterized by dysplastic or ineffective haemopoiesis that leads to variable anaemia, neutropenia, and thrombocytopenia. There is often a degree of red-cell macrocytosis. The majority are clonal stem cell disorders in which the abnormal clone predominates and expands only slowly over a number of years. Myelodysplasias have a tendency to develop ultimately into acute leukaemia in some patients; for this reason, they are sometimes referred to as ‘preleukaemias’, even though two-thirds of patients will never develop this complication. This chapter addresses the causes, diagnosis, and management of myelodysplastic syndromes.