This book provides an overview of various drug delivery systems at the cellular level including biological, chemical methods, and most importantly physical methods such as photoporation, electroporation, mechanoporation, and device-based techniques (e.g., microfluidics), as well as organism-level techniques including nanomaterials, biomaterials, and transdermal. Drug delivery (DD) can be defined as the method and route by which an active pharmaceutical ingredient (API) is administered to promote its desired pharmacological effect and/or convenience and/or to reduce adverse effects. Drug delivery systems are developed to maximize drug efficacy and minimize side effects. As drug delivery technologies improve, the drug becomes safer and more comfortable for patients to use. During the last seven decades, extraordinary progress has been made in drug delivery technologies, such as systems for long-term delivery for months and years, localized delivery, and targeted delivery. The advances, however, will face the next phase considering the future technologies that we need to overcome many physicochemical barriers for new formulation development and biological unknowns for treating various diseases. Thus, various technologies are built at a single-cell level as well as an organism level. This book is useful at the university level for graduate courses or research studies and biotechnology-based companies with research and development on cell-based analysis, diagnosis, or drug screening. This book is also very useful for researchers in drug delivery technologies, which came in frontier research for the past decade.