A FEW YEARS AGO there would have been little need to distinguish between the two halves of my title: John H. Van Vleck and magnetism, as a field of theoretical physics, were practically synonymous. Now the field has expanded so much that no one man can overwhelm all of its branches in the way Van did when my generation was being introduced to it. While I can safely assume that all physicists know some parts of Van's career in magnetism, I suspect few appreciate the whole of it.