Chapter 12 turns from heterolytic C–C bond formations in vivo to homolytic pathways at C–H and C–C bonds that involve carbon-centered radical species rather than carbanions and carbocations. One extreme is reaction of alkane and alkene carbons with high valent oxo-iron species in the active sites of oxygenases. O₂ is an obligate one electron acceptor in chemical biology. In oxygenase active sites high valent iron species cleave C–H bonds of bound substrates by hydrogen atom transfer to yield carbon radicals that are capturable by an [OH˙] equivalent in a radical rebound step. At the other end of the oxygen spectrum, substrate radicals occur under anaerobic conditions. More than 700 000 predicted protein open reading frames are proposed to cleave bound S-adenosylmethionine homolytically. The resultant 5′-deoxyadenosyl radical initiates C–H bond cleavage in a nearby bound cosubstrate to set off substrate radical chemistries.