Despite massive efforts in drug discovery fueled by combinatory chemistry, recombinant DNA technology, and high-throughput screening, surprisingly few molecules make it through the drug development process. While the reasons are debated, it is certain that many new chemical entities (NCEs) suffer from recurring problems that hinder developmentβlow water solubility, instability, or inadequate pharmacokinetics. The purpose of this book is to survey efforts to adapt, modify, or tailor polymers for challenging molecules. This book focuses on four classes of polymers in drug deliveryβwater-soluble polymers, hydrogels, biodegradable polymers, and assembled polymers. In this context, this book highlight efforts in the delivery of poorly absorbed molecules, poorly water-soluble or toxic molecules, peptide and proteins, and plasmid DNA. An attempt in this book has been made to highlight promising efforts in polymer synthesis, building structure-property correlations for drug delivery, and polymer assembles, which hold promise as nanoscopic carrier systems for a variety of biological molecules.