Six-membered heterocycles containing nitrogen and oxygen are prevalent in natural products and pharmaceutical drugs; hence efficient synthesis of these motifs is of great benefit. This thesis describes the successful rearrangements of 5-membered heterocycles containing nitrogen and oxygen through to the corresponding 6-membered compounds. Chapter 1 - Project Conception This section outlined the project conception and aims. Chapter 2 - Radical Ring-Expansions This section contains an introduction to the radical mediated ring-expansions, which firstly focused on carbocylic precedent, before an in depth discussion on work relating to the radical ring-expansion of heterocycles. Chapter 3 - Synthesis of Piperidines and Pyrrolidines An introduction to the rearrangement of nitrogen containing heterocycles was examined, with particular emphasis on the one carbon ring-expansion of pyrrolines, azetines and lactams. Chapter 4 - Rearrangements of Nitrogen Heterocycles This section detailed the one-carbon radical ring-expansion of the pyrrolines obtained from the Birch reduction of electron deficient pyrroles. The scope of the reaction was examined with respect to substitution around the pyrroline ring. A postulated mechanism was presented with experimental evidence in the form of the synthesis fused bicycles. Attempts at inducing either an ionic or cationic ring-expansion were examined and a novel pyrroline ring-opening reaction presented. Chapter 5 - Synthesis of Tetrahydropyrans The literature on the ring-expansion reactions to form tetrahydropyrans was discussed, with particular emphasis paid on the tetrahydrofuran to tetrahydropyran rearrangement, the ringexpansion of epoxides and the ring-expansion of thiiranium ions. Chapter 6 - Rearrangements of Oxygen Heterocycles The development of the ring-expansion of tetrahydrofurans to tetrahydropyrans was described, with focus centred on the stereochemical requirements for efficient ring-expansion. A hydride shift pathway to form open chain ketones was exploited in the enantiopure synthesis of spiroketals.