Book excerpt: This book elaborates on drug delivery targeting via intracellular delivery, specifically through the Receptor Mediated Endocytosis (RME) approach, due to the involvement of cellular receptors in various grave diseases. Targeted delivery relies on two basic approaches, passive and active targeting. While passive targeting approaches have shown great promise, the improved selectivity achieved with active targeting approaches has resulted in significantly higher efficacy. Interestingly there are numerous strategies for active targeting, many of which are already highlighted in , Targeted Drug Delivery: Concepts and Applications. Nevertheless an exciting and practical strategy for active targeting, which could enable high intracellular delivery, is through exploitation of RME. Cells in the body express receptors to enable various physiological and biochemical processes. As a result, many of these receptors are overexpressed in pathological conditions, or newer receptors expressed due to defective cellular functioning. RME is based on exploitation of such receptors to achieve intracellular delivery. While targeted delivery can have manifold applications, in this book we focus on two major and challenging therapeutic areas; i) Cancer and ii) Infectious Diseases. Targeted Intracellular Drug Delivery by Receptor Medicated Endocytosis discusses the major receptors that are useful for targeted delivery for these afflictions. A major section of this book is dedicated to details regarding their occurrence and location, the recognition domain of the receptor, structure activity relationship of substrate /ligand for selective binding, ligands explored, antagonists for ligand binding and relevance of these aspects for therapy of cancer and infectious diseases. These facets are elucidated with the help of specific examples from academic research and also emphasize commercial products, wherever relevant. In vitro cellular models relied on for assessing receptor mediated cellular targeting and in vivo models depicting clinical efficacy are focused on in a separate section. Finally, we briefly discuss the regulatory and toxicity issues that may be associated specifically with the RME approach of intracellular drug delivery.