The chemical literature does not strongly differentiate between photocatalytic (PC) and photosynthetic (PS) reactions (including artificial photosynthesis), even though these are different processes. Photocatalytic processes are thermodynamically downhill (ΔG<0) and are merely accelerated by a catalyst whereas photosynthetic processes are thermodynamically unfavorable (ΔG>0) and require photochemical energy input to occur. Here we apply this differentiation to formulate design criteria for PC and PS devices, with a special emphasis on solar water photoelectrolysis / solar hydrogen generation. As will be shown, under conditions of optimal light absorption, carrier lifetimes, and electrochemical rates, the performance of PCs is limited only by their surface area, while type 1 PS devices are limited by their carrier mobility and mass transport, and type 2 PS devices are limited by electrochemical charge-transfer selectivity. Strategies for the optimization of type 1 and 2 photosynthetic devices and photocatalysts are also discussed.