Colloidal semiconductor nanocrystals are remarkably versatile materials that exhibit a high degree of tunability in electronic structure, optical spectra, and surface properties. They are strong light absorbers and have potential applications in solar energy harvesting. This seminar will focus on the relationships between photophysics and photochemistry of nanoscale semiconductors with particular emphasis on light-driven reactions involved in solar water splitting. We have developed nanostructures that couple light harvesting II-VI nanocrystals with redox catalysts that perform proton reduction and water oxidation. The functionality of such hybrid structures depends strongly on the charge transfer dynamics between the photoexcited nanocrystal and the catalyst. We use ultrafast transient absorption spectroscopy to examine the dynamics of nanocrystalcatalyst charge transfer and the competitiveness of this process with energy-wasting charge recombination. I will discuss our recent findings identifying kinetic bottlenecks for photochemistry and describe our ongoing efforts to design nanocrystal structure and surface chemistry to enhance the competitiveness of the photochemical pathways.