Chronic myelomonocytic leukaemias (CMML) are myeloid neoplasms characterized by a sustained increase in monocyte counts in the peripheral blood, accompanied by dysplasia, abnormal proliferation, chromosomal anomalies and somatic mutations of haematopoietic cells. More than 95% of CMML patients harbour somatic mutations. CMML must be separated from other myeloid neoplasms and reactive monocytosis. The clinical presentation of CMML varies, but most frequently shows signs and symptoms of haematopoietic insufficiency or myeloproliferation. Robust instruments are available for assessing the prognosis of patients with CMML, such as the CMML-specific prognostic scoring system molecular. Treatment options for patients with CMML are still inadequate and generally less effective than those for other myeloid neoplasms. The only curative approach is allogeneic stem cell transplantation. This article explains essential aspects of CMML pathophysiology and provides an overview of diagnostic considerations, prognostic assessment and therapeutic options.