Development of polymeric gas separation membranes is one of the fastest growing areas of membrane technology. However, polymeric materials are somewhat deficient in meeting the requirements need for current membrane technology. Mixed matrix membranes (MMMs), formed from dispersion of rigid permeable or impermeable particles in continuous polymeric matrixes, present an interesting approach for improving the separation properties of existing polymeric membranes. Indeed, using excellent properties of both organic and inorganic phases makes it possible to prepare a membrane with good permeability, selectivity, mechanical strength, thermal stability, chemical resistance, and processibility. In this book, at first, different morphologies of the MMMs are discussed by considering their pros and cons. Afterwards, predictive permeation models for the MMMs are highlighted for different morphologies and then the potential of some permeation models in prediction of gas permeability in MMMs are validated with experimental data.