Cyclic structures, including both carbocycles and heterocycles, are common motifs of natural products and synthetic compounds with important biomedical activities. Among different approaches for preparing cyclic molecules, radical cyclization represents one of the most powerful approaches for construction of ring structures. Among a number of inherent synthetic advantages, radical reactions typically proceed at fast reaction rates under mild and neutral conditions in a broad spectrum of solvents and exhibit significantly high functional group tolerance. Furthermore, radical processes have the capability to perform in a cascade fashion, allowing for the rapid construction of complex molecular structures with generation of multiple stereogenic centers. To further enhance the synthetic applications of radical cyclization, new approaches will be needed for achieving high control of their reactivity as well as stereoselectivity, especially enantioselectivity, challenging issues that are intrinsically associated with the “free” nature of radical chemistry. Guided by the concept of metalloradical catalysis (MRC), this proposed research explores a fundamentally new approach for controlling stereoselectivity of both C- and N-centered radical reactions. Cobalt(II) porphyrins [Co(Por)], as stable 15e metalloradicals, can enable the activation of diazo compounds and organic azides to cleanly generate C- and N-centered radicals, respectively, with N2 as the only byproduct in a controlled and catalytic manner. The initially-formed C- and N-centered radicals, which are termed as a-Co(III)-alkyl radicals and a-Co(III)-aminyl radicals, respectively, remain covalently bonded with [Co(Por)]. They can undergo common radical reactions, such as radical addition to alkenes and hydrogen-atom abstraction, but with effective control of reactivity and stereoselectivity by the porphyrin ligand environment. In addition to the radical nature of [Co(Por)], the low dissociation energy of Co–C and Co–N bonds plays a key role for the successful turnover of the Co(II)-based catalytic processes, resulting in effective radical cyclization reactions. Through the support of D2- symmetric chiral porphyrin ligands with tunable electronic, steric, and chiral environments, Co(II)-based metalloradical catalysis (Co(II)-MRC) will be applied for the development of various radical cyclization processes for stereoselective construction of both carbocylic and N-heterocyclic compounds with different ring sizes and varied degrees of molecular complexity. We hope these studies will ultimately lead to the development of cost-effective and environmentally benign radical cyclization processes that can be successfully applied for the stereoselective synthesis of biologically important natural products and pharmaceutically interesting small molecules.