The present dissertation seeks to deepen our understanding of hydrogen transfers and especially C-H bond activations in enzymes. Hydrogen transfers are ubiquitous in chemistry and biology and a thorough understanding of how they occur and what factors influence them will facilitate developments in biomimetic catalysis, rational drug design, and other fields. A particular difficulty with H-transfers is the importance of nuclear quantum effects to the reaction, particularly tunneling. The overall scope of the work here aims to examine how experimental kinetic isotope effects (KIEs) can be interpreted with a particular type of tunneling model, referred to as Marcus-like models, to yield a semi-quantitative picture of the physical mechanisms of H-transfers. Previous work had used this kind of model to qualitatively interpret experimental data using a combination of intuition and generalized theories. The work here examines these theories in quantitative detail, testing and calibrating our intuition in the context of several experimental systems. The first chapter of research (ch. II) focusses on the temperature dependence of primary KIEs and how these experiments can be quantitatively interpreted as a probe for certain kinds of enzyme or solvent dynamics. The subsequent chapters (ch. III-VI) focus on the use of secondary KIEs to determine the detailed structures of tunneling ready states (TRSs) and how the dynamics of H-tunneling affect those structures. These chapters focus primarily on the TRS of the enzyme alcohol dehydrogenase, but by examining an uncatalyzed analogue to that reaction (ch. VI), the work gains some insight about similarities and differences between catalyzed and uncatalyzed reactions. In summary, the work uncovers some principles of catalysis, not just the mechanism of a catalyzed reaction. The mechanism of C-H activation presented here provides an elegant solution to problems that have been vexing to accommodate within traditional models. This work constitutes some initial steps in making Marcus-like models quantitatively useful as a supplement or even replacement for traditional models of reactivity.