As solar energy is the major driving force of the global cycles of key elements including C, N, O, and H. Photo (electro) catalysis is an ideal engineering method to mimic and restore the nature’s element cycles. This talk will introduce our recent efforts on photocatalytic and photoelectrochemical (PEC) conversions of common C (CO2), N (NO3−, NO2−, NO, NH4+, and N2), and O (O2, H2O2) compounds and discuss their environmental implications.