Singlet fission is a photophysical process that has promise for increasing the efficiency of solar cells. The dynamics depend on triplet spin states and can be influenced by external magnetic fields. In 4-electron systems, fission takes an initial singlet state into a superposition of triplet pair states. Direct evidence for this superposition state is provided by quantum beats in the delayed fluorescence of tetracene crystals. The beat frequencies depend on crystal orientation with respect to the magnetic field, consistent with predictions based on solving the full spin Hamiltonian. Magnetic field effects on the kinetics are analyzed in terms of a hybrid quantum-kinetic model. The magnetic field has no effect on the initial fluorescence decay rate but affects the decay after the triplet pair states begin to equilibrate with the singlets. The long-time behavior of the fluorescence decay reflects association and separation of triplet pairs and relaxation into different spin states. (C) 2013 Elsevier B.V. All rights reserved.