Single unit monomer insertion (SUMI) is an emerging technology for precise polymer synthesis through a radical addition reaction mechanism with monomer additions occurring one at a time. It possesses the capability of highly efficient and economical chemical conversion for carbon-carbon bond formation. Originating from reversible deactivation radical polymerization (RDRP), SUMI retains the virtues of mild reaction conditions and remarkable tolerance toward functionalities. Additionally, SUMI can provide a versatile platform for the engineering of stereochemistry as well as mechanistic and kinetic studies of radical addition reactions and RDRP. Herein, the history and development of SUMI are reviewed, and the advances since its advent in organic transformation and sequence-defined polymer synthesis are highlighted. Current challenges are discussed, and a perspective on future opportunities for this promising synthetic technology is also presented.