One of the most important obstacles to successful oral drug delivery is the poor solubility of many drugs in aqueous media. This problem is of intense scientific and practical interest, impacting the effectiveness of marketed drugs, as well as the success rate and expense of new drug development. A number of solubility enhancing techniques have been developed over recent decades, including complexation with cyclodextrins, reducing particle size, forming lipid dispersions, and creating intimate mixtures of drug and polymer in the solid state (amorphous solid dispersion, or ASD). Polymers (and in the case of cyclodextrins, oligomers) are indispensable to each of these approaches. We describe in this review the design and function of polymers in each of these solubility enhancement methods, highlighting structure-property-function relationships, pointing out the advantages and disadvantages of each approach, and describing the functional demands upon current and future polymers. We conclude by discussing unresolved issues that may be addressed in the future by polymer scientists. (C) 2015 Elsevier Ltd. All rights reserved.