Aplasia is a pathologic term that is broadly defined as the absence or near-absence of one or more haematopoietic lineages in the bone marrow (BM). Clinically, BM aplasia affecting more than one lineage is referred to as aplastic anaemia (AA), despite the fact that this group of disorders often results in pancytopaenia rather than anaemia alone. Cytopaenias can be seen in a number of different conditions, and new-onset pancytopaenia in children and adults requires an extensive work-up, including a BM core biopsy (BMB) for confirmation of haematopoietic aplasia/hypoplasia and exclusion of an infiltrative marrow process or fibrosis. Bone marrow aplasia develops as a result of injury to multipotent haematopoietic stem cells, which can occur in the context of constitutional (primary aplasia) or acquired (secondary aplasia) disorders (Table 4.1). This chapter will discuss the diagnostic criteria and pathophysiology of specific disorders presenting with aplasia and demonstrate an algorithmic approach to the diagnostic evaluation of patients presenting with this common and non-specific finding (Table 4.2).