Electron transfer (ET) pathways govern our world on one of the most fundamental levels, determining the outcomes of countless complex chemical processes that build our society. Controlling ET and driving chemical product formation effectively, however, is a substantial challenge. Coupling semiconductor nanocrystals with redox enzymes enables opportunities to drive complex, multielectron chemistry with a renewable solar energy source, while at the same time opening up a world of possibilities to study the fundamental principles behind the control of ET in these systems. This dissertation investigates ET pathways and catalysis in photoexcited CdS nanocrystals and CdS nanocrystal-[FeFe] hydrogenase biohybrid complexes. Each of the four projects described here focuses on ET in a different part of this system: 1) Interfacial nanocrystal-to-hydrogenase ET, with transient absorption spectroscopy and photochemical H₂ production with a size series of CdS quantum dots (QDs). We find that QD properties other than size, likely QD surface properties, have the greater influence on the photocatalytic outcomes here. 2) The kinetics of ET within a hydrogenase photoreduced by CdS QDs. We model the resulting hydrogenase redox species’ kinetics from photochemical electron paramagnetic resonance spectroscopy experiments to gain a picture of the thermodynamic landscape of the intraprotein ET pathway. This work provides insight into the properties of the hydrogenase iron-sulfur clusters and organometallic active site that tune the directionality of ET and catalysis in this enzyme. 3) Examination of the nanocrystal-hydrogenase photochemical system as a whole, using experimental H₂ production trends and kinetic Monte Carlo simulations to reveal how key parameters such as hole transfer, catalyst turnover, and back-transfer processes determine rate-limiting conditions. 4) Investigation of a charge transfer pathway in CdS nanocrystals which, under certain conditions, leads to long-lived photoreduced states even in the absence of an external electron donor. We discuss the conditions under which this process occurs, and its relevance for other CdS photochemistries.