Atom transfer radical polymerization (ATRP) is a catalytic process mediated by a redox-active transition metal complex able to abstract homolytically a halogen atom from an alkyl halide in a reversible fashion. The mechanistic understanding of ATRP is crucial and enables the rational selection of reaction components and conditions needed for the synthesis of well-defined polymers. This chapter provides an overview of the parameters that determine the activity and performance of the ATRP catalyst, i.e., its ability to control the polymerization and also its propensity to participate in undesired side complexation or redox reactions with reaction components (monomer, polymer, propagating radicals, solvent, and other additives). The effects of the central atom or ion of the catalyst, the ligand, the transferable halogen atom, and the reaction solvent can be described quantitatively by formally splitting the overall atom transfer equilibrium into contributing reactions, including the homolysis of a carbon–halogen bond, reduction of the halogen atom to a halide ion (electron affinity), oxidation of the lower oxidation state metal complex (activator),and formation of the radical deactivator via coordination of a halide anion to the higher oxidation state metal complex (halidophilicity). Additionally, the factors that are responsible for the activity of alkyl halides as radical sources, including structure of the alkyl group and the nature of the halide are also discussed.