This chapter focuses on basic introduction to the haematopoietic system and the bone marrow and erythroid series of disorders. The bone marrow provides a suitable environment for growth and development of stem cells. The stem cells express a variety of cell surface proteins such as CD34 and adhesion proteins which help the stem cells to “home” to the bone marrow when infused. The stem cells have the appearance of small or intermediate sized lymphocytes and their presence in the marrow can be demonstrated by cell culture techniques by the growth of colony-forming units (CFU) pertaining to different cell lines. Myeloid haematopoiesis or myelopoiesis includes differentiation and maturation of granulocytes, monocytes, erythroid cells and megakaryocytes. Bone marrow aspiration involves suction of marrow via a strong, wide bore, short-bevelled needle fitted with a stylet and an adjustable guard in order to prevent excessive penetration. Trephine biopsy is performed by a simple Jamshidi trephine needle by which a core of tissue from periosteum to bone marrow cavity is obtained. The megaloblastic anaemias are disorders caused by impaired DNA synthesis and are characterised by a distinctive abnormality in the haematopoietic precursors in the bone marrow in which the maturation of the nucleus is delayed relative to that of the cytoplasm.