As shown, there are now an abundance of methods one can choose from, each with their own profits and consequences. In colloquial terms, one now has 57 ways to skin the same cat . But which is correct for a given system? This comprehensive review will examine the various theories used to calculate internal conversion in numerous systems and phases of matter. Beginning with the early success and later deprecation of pioneering work by Lin, Siebrand, and Robinson-Bixon-Jortner, internal conversion is examined in the context of Fermi's Golden Rule, Arrhenius-like methods, and the Plotnikov-Robinson-Bixon-Jortner models for molecular systems. We also consider more complex cases such as internal conversion in the solution phase, condensed matter systems, and quantum dot and nanostructures. We then move away from molecules and begin to examine more complex systems such as those in complex solution and condensed matter. Finally, internal conversion will be explored in the context of more niche applications, such as in exciton-polariton coupled systems, deuteration effects, and exact conical intersections.