The reactivity of radical polymer chains and transition metal complexes is analyzed, highlighting the contribution of computational investigations. The main phenomena are halogen atom transfer in atom transfer radical polymerization (ATRP), direct bond formation in organometallic-mediated radical polymerization by the reversible termination strategy (OMRP-RT), associative radical exchange in OMRP by the degenerative transfer strategy (OMRP-DT), β-H atom transfer to generate a hydride complex intermediate in catalytic chain transfer (CCT), and reaction with the OMRP dormant species in catalyzed radical termination (CRT). The introductory part highlights the principles of controlled polymer growth and the necessary conditions leading to low-dispersity, high chain-end fidelity polymers. Later sections are dedicated to the various phenomena that occur in controlled polymerizations taking place by a pure ATRP, pure OMRP (RT or DT), or interplaying ATRP/OMRP mechanism. The phenomena that interfere with controlled chain growth (CCT and CRT) are also covered in separate sections.