In planning studies on coronary heart disease and in evaluating the results of such research, care must be taken to understand exactly which aspects of the disease complex are being investigated and where the results fit into the complete picture. The nomenclature is extensive, each term has an exact meaning and the terms should not be used synonymously. Although patients with angina pectoris are most suitable for study, accurate selection of subjects is difficult in the absence of objective diagnostic tests. Furthermore, angina pectoris represents only one aspect of coronary heart disease and facts pertaining to one phase are not necessarily applicable to the whole. The electrocardiogram is of great value, but abnormalities often considered as diagnostic are not pathognomonic of coronary heart disease. In the evaluation of therapy the facts that patients differ in their ability to respond and that drugs vary in their effectiveness must be taken into account. Studies in laboratory animals have definite value, but usually measure only one of the many interrelated physiological factors involved in coronary heart disease; as such the results cannot be applied directly to the treatment of angina pectoris.