The author made extensive studies on 635 subjects varying in age from 15 minutes to 16 years; the usual bipolar and unipolar leads being used. Normal standards, for each of the deflections and intervals in the various leads, were established for this age group. These are presented by means of 33 tables, 43 illustrations (electrocardiograms), and an appendix (62 pages) of statistical data that can be understood and used by the average clinician. The method of presentation helps one to visualize the progression of changes through infancy and childhood. Information is included regarding the changing ratio of the mass of the right as compared with that of the left ventricle during growth and development and for the accurate determination of abnormal degrees of ventricular enlargement at all ages. There is a simple discussion of fundamental principles and current concepts of electrocardiography. An adequate review of the literature, a carefully selected