Risk of disease and response to treatment varies from person to person. This is due to variation in human genetic coding, interactions between one's genes and environment over a lifetime and the unique signature of the immune system. Defining the scope and nature of human biological variation allows the targeting of medical treatments to those most likely to benefit. Such treatments may include drugs or cell therapies tailored to a patient's history, genes and immunology. This POSTnote examines the state of research into human variability, and the prospects, challenges and policy implications of more personalised medical treatment.