The essential problem of malignancy is associated with the nature of the disturbance which leads to the purposeless multiplication and spread of cells in the body of the afflicted individual. Before overt manifestations of malignancy - cancer or leukaemia - are discernable, a whole series of events takes place some of which can be investigated either clinically or experimentally. The most interesting objects of current research are the identification of a causative agent and the kinetics of the proliferation of the transformed cells at the expense of the normal cells of the body. The knowledge gathered from these investigations will hopefully provide a basis for the institution of a therapy which results in lasting remissions while avoiding the damage of normal cells such as occurs with the present forms of treatment.