Owing to the prevalence of C=C bonds in feedstock chemicals and synthetic intermediates, the difunctionalization and heterodifunctionlization of alkenes provide an efficient strategy for rapidly increasing the complexity of molecules in organic synthesis. Over the past four years, my work in Lin lab has primarily focused on methodology development of transition-metal-catalyzed difunctionalizaiton and hydrofunctionalization reactions of C=C bonds. A diverse range of modes of activation including electrochemical, photochemical, and chemical methods to access open-shell intermediates have been employed towards this purpose. In this dissertation, we begin with a brief overview of existing strategies of alkene functionalization. We then discuss some common features of these strategies and key advancements and challenges in this area. We are particularly interested in leveraging the unique properties of radical intermediates to access novel alkene functionalization reactions. An electrocatalytic vicinal diazidation of alkenes with manganese porphyrin complexes was first demonstrated. This protocol shows improved practicality over our previous work using MnBr2 catalysis in the following aspects: (1) it requires substantially lower catalyst loading; (2) the introduction of a neutral aqueous buffer prevents the generation of toxic hydrazoic acid, contributing to a safer experimental procedure; (3) the catalytic system displays improved reactivity towards unactivated terminal alkenes. Mechanistic studies support the roles of second-sphere hydrogen-bond donors in stabilizing key reaction intermediates. Next, we explored the area of hydrogen-atom transfer reactions to alkenes mediated by CoIII-H. Current methods to generate CoIII-H most frequently rely on oxidatively initiated hydride transfer. In order to address some limitations associated with this method, we develop a reductive approach to generate CoIII-H, which allows for canonical hydrogen evolution reactions to be intercepted by hydrogen-atom transfer to an alkene. Electroanalytical and spectroscopic studies provided mechanistic insights into the formation and reactivity of CoIII-H, which enabled the development of alkene deuteration and hydroarylation reactions. Finally, mechanistic insights gained from the study of CoIII-H led to the development of the hydrofluorination of unactivated alkenes. Alkene hydrofluorination reaction represents an attractive strategy for the synthesis of aliphatic fluorides. This approach provides a direct means to form C(sp3)–F bonds selectively from readily available alkenes. Nonetheless, conducting hydrofluorination using nucleophilic fluorine sources poses significant challenges due to the low acidity and high toxicity associated with HF and the poor nucleophilicity of fluoride. We present a new Co(salen)-catalyzed hydrofluorination of simple alkenes utilizing Et3N·3HF as the sole source of both hydrogen and fluorine. This process operates via a photoredox-mediated polar-radical-polar crossover mechanism. We also demonstrated the versatility of this method by effectively converting a diverse array of simple and activated alkenes with varying degrees of substitution into hydrofluorinated products. Furthermore, we successfully applied this methodology to 18F-hydrofluorination reactions, enabling the introduction of 18F into potential radiopharmaceuticals. Our mechanistic investigations, conducted using rotating disk electrode voltammetry and DFT calculations, unveiled the involvement of both carbocation and CoIV–alkyl species as viable intermediates during the fluorination step, and the contribution of each pathway depends on the structure of the starting alkene.