The concept of collection development is central to the professional practice of librarianship as the whole notion of a library is fundamentally associated with the idea of a collection, to the extent that the words 'library' and 'collection' are almost synonymous. Other terms such as 'information centre' 'learning centre' or 'discovery centre' are often used now instead of 'library' and terms such as 'information resources' may be substituted for 'library collections', but the activities and processes traditionally associated with collection development are still essential to the effective functioning of contemporary library, information, learning and knowledge services, even though they may look and feel quite different to their historical counterparts. Our concern here is to explore the concept of collection development in the digital library and information environment and in particular to examine how thinking and practice in this vital area of library and information management have developed and changed in response to advances in digital technologies. What are the similarities and differences between collection development yesterday and today? Investigating such questions should help to prepare us for dealing with the collections of tomorrow.The chapter therefore starts by examining a few definitions of the library as a collection to demonstrate the centrality of collection development to library and information professionals. It moves on next to the more problematic issue of collection development as a concept, showing how professional discourse on the subject has suffered from confused terminology. It then uses a convenient four-phase framework to review the impact of information and communication technology (ICT) on libraries, concentrating on effects on collections and their development. The final part of the chapter returns to academic and practitioner conceptions of collection development in digital environments, concluding with a set of questions for reflection on the future of collection development.