Originally published in 1967, this volume contains the proceedings of the first international conference held to discuss the triplet state and the various physical phenomena known to be associated with molecules having orbital electrons with parallel spins, and techniques for their investigation. A number of results and ideas in the field of magnetic resonance and phosphorescence of the triplet state are reported for the first time. The topics covered are spin-orbit coupling, intersystem crossing, magnetic resonance, magnetic interactions, photochemistry, radiationless transitions, triplet excitons, delayed fluorescence, phosphorescence and the triplet state related to biology. This volume brought together knowledge concerning the triplet state and defined the problems in need of solution. The review nature of the plenary papers and the good coverage of the volume as a whole made it a basic reference book on the subject, which is still of relevance today.