This book provides detailed information on the base catalysis of group 5 (Nb, Ta) metal oxide clusters by elucidating how the structural factors such as constituent metals, counter cations, and local structures of base sites affect their catalysis. Uniquely, it reveals the effects of key structural factors at the molecular level by combining experimental and theoretical approaches. The findings presented here provide rational design principles for base catalysis and will foster the development of promising catalysts for solving current and future energy and environmental problems.