Bone marrow (BM) hyperplasia, a non-neoplastic expansion of one or more of the haematopoietic cell lineages due to an increased number of cells, can manifest in a range of morphological appearances depending on the underlying cause. Similarly to other tissue types, hyperplasia is often associated with an increase in the number of cells with less mature morphology. It is this reactive atypia/dyshaemopoietic morphology that needs careful assessment and correlation/integration with clinical, biochemical, radiological and often molecular findings to correctly interpret the underlying process and avoid misdiagnosis as a neoplastic proliferation. This chapter will consider erythroid, myeloid and megakaryocytic hyperplasia. Reactive conditions of histiocytes are covered in Chapter 6.