The myeloproliferative neoplasms (MPNs) are clonal haematopoietic disorders characterized by increased numbers of mature myeloid cells. Polycythaemia vera (PV), essential thrombocythaemia (ET) and primary myelofibrosis (PMF) are linked by their associations with mutations in the JAK2 gene, and for ET and PMF in CALR and MPL , and by their risks of thrombosis, haemorrhage and disease transformation, including to acute myeloid leukaemia. Although all three disorders are associated with reduced life expectancy, PV and ET have a more chronic course and are managed primarily according to vascular risk, with cytoreductive drugs employed in higher risk patients. Primary myelofibrosis carries a less favourable prognosis, influenced by clinical and genomic factors; therapy is risk-stratified accordingly and may include allogeneic stem cell transplantation in younger high-risk patients, JAK inhibitors and other symptomatic treatments. Mastocytosis comprises a heterogeneous group of rare neoplasms in which clonal mast cells accumulate in the skin or extracutaneous organs, strongly linked to mutations in the KIT gene. For most patients therapy is symptomatic, relying on avoiding factors that trigger mast cell mediator release and management of resulting symptoms, although patients with more advanced disease may warrant therapy to reduce the mast cell burden including KIT inhibitors. This chapter also covers non-clonal causes of erythrocytosis and thrombocytosis, as well as some rare clonal causes of eosinophilia and neutrophilia.