The direct functionalization of inert aliphatic C–H bonds is a valuable approach for synthesizing pharmaceuticals, agrochemicals, and other functional molecules from readily available starting materials. This strategy is particularly useful in drug discovery, where late-stage diversification of drug-like molecules can produce diverse chemical entities from existing libraries, facilitating structure-activity relationship studies and circumventing laborious de novo synthesis. Despite its potential, achieving site-selective functionalization of strong and prevalent aliphatic C–H bonds remains a significant challenge, especially in the absence of directing groups. In this dissertation, we first provide an overview of current C–H functionalization methods, with an emphasis on hydrogen atom transfer strategies. We then reviewed the history and applications of frustrated radical pairs in chemical bond activation and organic synthesis. Later, we developed a novel approach for the aminoxylation of C(sp3)–H bonds using frustrated radical pairs derived from an alkali base donor and an oxoammonium acceptor. Furthermore, we achieved regiodivergent C–H functionalization through structural modification of the alkali base donor. Finally, we developed a new hypervalent iodine reagent that releases a potent hydrogen atom abstractor for C–H activation under mild photochemical conditions. Using this reagent, we demonstrate the selective (N-phenyltetrazole)thiolation of aliphatic C–H bonds across a broad range of substrates. Additionally, we accomplished the diversification of C(sp3)–H bonds into C–S, C–Cl, C–Br, C–I, C–O, C–N, C–C and C=C bonds by simply altering the radical trapping agent.