Abstract: Spin plays a key role in organic semiconductors, with examples ranging from spin-dependent losses in organic photo- voltaics and light-emitting diodes to magnetic field-dependent charge mobility in transistors. One such spin-dependent process, singlet fission, involves the production of two triplet excitons (each with spin S=1) following excitation of one singlet exciton (spin S=0). This pair production process has the potential to boost the theoretical efficiency of photovoltaics beyond the Shockley-Queisser limit, while the reverse process, triplet-triplet annihilation, underpins applications of organic semiconductors in photocatalysis, bio-imaging, and light-emitting diodes. Recently we have used singlet fission as a means of optically generating spin-1 excitations to study their fundamental spin interactions. We have deployed broadband optically detected magnetic resonance, electron spin resonance, and magneto-optics to extract dipolar and exchange interactions between triplet excitons in an organic semiconductor. Mapping the experimentally extracted spin parameters onto the molecular crystal structure provides a window into exciton localization, dissociation, and diffusion in the molecular lattice. This mapping of spin properties to excited-state electronic structure is made possible by sustained excited-state spin polarization and coherence over microsecond timescales, and so these spin properties not only provide us with a probe of electronic processes in organic materials, but also present an opportunity for future development of molecular spin-based technologies.