Polymer blend technology and gas membrane separation have been two research areas where Prof. Don Paul has had major contributions. The combination of these topics has been chosen as the subject of this review paper. The morphology of phase separated polymer blends is an important variable in the design of enhanced transport properties. The fundamentals of polymer blends in transport processes will be discussed along with situations where polymer blends may be desired. These areas include barrier polymers, gas membrane separation, percolation networks for electrical conductivity, proton exchange membranes, and polymers employed in optoelectronic devices. The predictions of the series, parallel, Maxwell, and equivalent box models are applied where relevant to these examples.