Atmospheric oxidation is characterized by a broad range of dierent uni- and bimolecular reactions. Unlike their bimolecular counterparts, the rate coecients of unimolecular reactions have been found to depend strongly on the structure of the reacting compound and thus the eect of these reactions in atmospheric oxidation is dicult to assess accurately.Yet, the competition between uni- and bimolecular reactions is expected to have important implications for the properties of the formed products in terms of oxidant recycling and formation of secondary organic aerosol (SOA), both of which are likely promoted by unimolecular reactions.A cost-eective computational multi-conformer tra nsition state theory (MC-TST) approach for calculating rate coecients of unimolecular reactions was developed and has been applied to a broad range of reactions in an attempt to assess the factors governing their reaction rate coecients. A major focus has been on peroxy radical hydrogen shift (H-shift) reactions in both acyclic and cyclic compounds and the large number of calculated rate coecients for this reaction class provides insight into the ranges of rate coecients that can be expected for dierent molecular motifs, akin to what would be possible from a structure-activity relationship (SAR). A combination of experiments and theory is used to show that stereoselectivity, the eect that dierent stereoisomers react with dierent rate coecients, may be signicant in the atmosphere. Implementation of calculated rate coecients into a global chemistry-transport model conrms that peroxy radical H-shifts are highly important in the oxidation of isoprene. The alkyl radicals that are typically the products of the unimolecular peroxy radical H-shifts may undergo further unimolecular reactions to form epoxides. However, these reactions need to be faster than the competing bimolecular reactions to be atmospherically competitive and have so far been thought to happen only for very specic systems. Based on a systematic theoretical study, we nd that epoxide formation is likely atmospherically competitive for systems with certain hydrogen bonding motifs and thus may be much more ubiquitous in the atmosphere than previously thought. Finally, we use theoretical methods to investigate an experimental enigma related to the dierence in product distributions in the nitrate-initiated oxidation of the two important monoterpene species, -pinene and -3-carene. For these systems, we nd that peroxy radical reactions are too slow to be of importance and instead, their product yields are governed by dierences in their alkoxy radical reactions. The results presented here broaden the understanding of the rate coecients of unimolecular atmospheric reactions and thereby aid the work of implementing these into atmospheric chemical models. In an ever warming environment with increasing focus on limiting NOx-emissions, the importance of most of these unimolecular reaction pathways is only expected to increase in the future, even in urban areas.