This thesis explores single-atom ring expansion reactions using reagents that deliver single atom equivalents. Chapter 1 reviews the current strategies in the literature for nitrogen atom transfer, with a specific emphasis on the reagents used for delivering nitrogen atoms in organic synthesis. Chapter 2 demonstrates a stoichiometric cycle for nitrogen atom transfer from a cationic osmium nitride to indenes, resulting in ring expansion to isoquinolines. Mechanistic studies on the nitrogen insertion step provide insights into the uniqueness of the metal nitride to enable the ring expansion, and regeneration of the nitride sets the stage for future catalytic efforts. Chapter 3 discusses a novel nitrogen atom transfer reagent based on dibenzoazabicycloheptadiene (dbabh) and its use in ring expansion reactions of indanones. This novel reagent enables two distinct ring expansion pathways depending on the mode of activation, allowing indanones to be converted to different nitrogen heterocycles under thermal and photochemical conditions. The mechanism of each transformation is discussed, and the different reactivity modes of the indanone-dbabh adducts are compared to other nitrogenous precursors. Chapter 4 explores the mechanism of chlorocarbene reactivity with azoles using Density Functional Theory calculations. Despite the similarities between pyrroles and pyrazoles, their reactivity with chlorocarbenes is shown in this chapter to be significantly different. Detailed calculations are used to compare the two related reactions and define the nature of each reaction mechanism.