Publisher Summary Theoretical techniques have revolutionized the study of chemistry. Due to the parallel development of theoretical methods and computing power, quantum chemistry has become fast and flexible, often yielding detailed insight into chemical processes that cannot be obtained in the laboratory. The earliest report of combining quantum chemistry with the atmosphere deals with the excitation of nitrogen and oxygen molecules as the primary event in the photo-excitation of the atmosphere. Models of atmospheric chemistry have grown with the abilities of computers and the demands of simulating climate change. One example is the simulation of isotopic fractionation in the photolysis of nitrous oxide isotopologues using time-dependent wave-packet propagation. Quantum chemistry provides data that improves understanding of chemical kinetics. The data is further used as input for parameterizing transport and deposition models or chemical reaction schemes in models of various other atmospheric processes. Theoretical techniques are tested through comparison to laboratory measurements and atmospheric observations, and then further applied towards predicting mechanisms and reaction rates which are currently unknown.