This 1984 book brings together 48 studies and essays written by physicists from around the world to mark the 90th birthday of Louis de Broglie and the 80th birthdays of Paul Dirac and Eugene Wigner. Publication under one cover of studies based on the discoveries of scientific personalities as diverse as those to whom this volume is dedicated serves at least one important purpose: it concretely reflects the rival pathways in quantum physics at the time of this book's publication. From one perspective, the papers contrast the strategies of a champion of the mathematical approach (Wigner) with those of a thinker who relied on physical intuition (de Broglie). From another, they compare the views of a defender (Wigner) and and opponent of the Copenhagen school (de Broglie), as well as those of an individual who helped to reconstruct the prevailing quantal paradigm, only later to advocate a return to causality (Dirac).