Digital libraries are collections of digital content and services selected by a curator for use by a particular user community. Digital libraries offer direct access to the content of a wide variety of intellectual works, including text, audio, video, and data; and may offer a variety of services supporting search, access, and collaboration. In the last decade digital libraries have rapidly become ubiquitous because they offer convenience, expanded access, and search capabilities not present in traditional libraries. This has greatly altered how library users find and access information, and has put pressure on traditional libraries to take on new roles. However, information professionals have raised compelling concerns regarding the sizeable gaps in the holdings of digital libraries, about the preservation of existing holdings, and about sustainable economic models. This chapter presents an overview of the history, advantages, disadvantages, and design principles relating to digital libraries, and highlights important controversies and trends. For an excellent comprehensive discussion of the use, cost and benefits of digital libraries see Lesk (2005), for further discussion of architectural and design issues see Arms (2000), and see Witten and Bainbridge (2002) for a detailed example of the mechanics of implementing a digital library.