Recent mechanistic innovations concerning catalyst development in atom transfer radical polymerization (ATRP) are described. The following topics will be discussed: structure-reactivity relationships of the polymerization catalyst, including correlating reaction parameters with catalyst, alkyl halide, and monomer structure; concurrent reactions that may occur during ATRP and will affect its efficiency, including oxidation/reduction of radicals to radical cations/anions, solvent, monomer, and radical coordination to the active catalyst, and side reactions particular to aqueous media. In addition, novel methods of fine tuning initiation, activation, and deactivation processes, including simultaneous reverse and normal initiation ATRP, activators generated by electron transfer ATRP, hybrid catalyst systems, and bimetallic ATRP will be presented. (c) 2006 Published by Elsevier B.V.