This chapter is a foray through the theoretical framework for both neutrino interactions with matter and nuclear reactions. The discussion of neutrino interactions is partitioned into charged-current and neutral-current formulations. The nuclear theory is stilted towards concepts that impact a nucleus’s journey to fission. This includes the nuclear binding energy, a macroscopic view of the nucleus found in the liquid drop model, and a microscopic view of the nucleus via the shell model. These orient the subsequent discussions of nuclear fission, its energy threshold, and various mechanisms through which the fission barrier can be lowered. Lastly, the many possible final state particles from nuclear fission are enumerated and described.