The field of haematology is relatively new, but the study of the blood has been going on for almost 400 years. The human body produces about two million red blood cells per second, and replaces 330 billion cells per day, however, disorders of the blood may be completely asymptomatic. Features of deficiency or excess of various blood constituents may only manifest at a late stage and are commonly vague and non-specific. Blood disorders themselves may be primary, due to an underlying issue with the bone marrow or component parts, or secondary, reflecting the impact of other disease processes on the haematological system (as shown for example by a disruption of Virchow’s triad). Nevertheless, careful history taking is essential to clarify the time course of events, precipitants and circumstances leading to the disorder. The speed of onset of symptoms for example is central to diagnosis and may impact the speed of referral, if needed, to specialist haematology teams.