Copper-catalyzed atom transfer radical polymerization (ATRP) is one of the most robust and precise techniques for controlling radical polymerization. The very good control of molecular weights, polydispersities, functionalities, chain composition, and topologies unusual for radical systems combined with the application of transition metals as catalysts requires more detailed mechanistic studies and proof of the radical nature of active species. The following results are in agreement with the radical nature of ATRP : reverse ATRP, chemoselectivities similar to those for conventional radical polymerization (effect of additives and inhibitors/scavengers, reactivity ratios, transfer coefficients), regioselectivities similar to those for conventional radical polymerization (low proportion of head-to-head units and expected structure of both tail and head end groups), stereoselectivities (tacticities) similar to that in conventional radical polymerization, EPR detection of X-Mt(n+1) species resulting from the persistent radical effect, and confirmation of the termination by doubling molecular weights as well as cross-linking with multifunctional initiators and inimers. In addition, it seems that, in most ATRP systems, the contribution of degenerative transfer and reversible formation of organometallic intermediates is small and a halogen atom is transferred in the concerted process rather than in a two-step process with the involvement of radical anions.