This book reviews recent advances in polymer swelling resulting from the use of novel microporous composite films. It offers a new approach to understanding sorption processes in polymer-liquid systems based on the molecular structures of the sorbed molecules and the repeat unit of the sorbent polymer. It is shown how the adsorption parameters obtained in these studies relate meaningfully with the Flory-Huggins interaction parameters. This implies that these adsorption parameters have relevance not only for swelling and drying of polymers, but also for other phenomena in which molecular sorption plays an important role, such as in chromatography and in membrane permeation.