This document discusses polymorphic nanocrystalline metal oxides and their thermodynamics and applications. It explains that nanocrystalline metal oxides exhibit polymorphic phase transitions at lower temperatures and pressures than their bulk counterparts due to effects of increased internal pressure and surface energy at the nanoscale. A thermodynamic model is presented to explain how surface area to volume ratio and internal pressure effects can lower the activation barrier for phase transitions. Examples of different polymorphic phases exhibited by various metal oxides at the bulk and nanoscale are provided. Applications of polymorphic nanocrystalline metal oxides in gas sensing, catalysis, solid oxide fuel cells, and electrochemical energy storage are discussed.