The thermodynamic theory of van der Waals and Platteeuw has served up to now and for many decades as the principal basis for the development of tools for predicting the equilibrium conditions of various gas hydrate systems. Several of these developments are briefly presented in the present paper, with the aim of revealing associated difficulties and possible inherent limitations. After a discussion of thermochemical properties, a general phase equilibrium model is presented in detail and is tested against experimental gas hydrate data of recently reported synthetic mixtures and petroleum reservoir fluids with and without methanol inhibitor, A critical discussion of relevant thermodynamic stability algorithms is also proposed. Finally, areas are indicated where further academic research might aid industrial practice.