In this article, the author discusses collection development. Collection development is the one professional activity that allows librarians to define the coin of their realms: the content and kinds of materials they can offer the patrons they serve. Collection development begins with a collection development plan that underscores the mission of both the school library and the school system within which it operates, the goals and objectives of the two organizations, and the role of the librarian. The collection development plan defines the patrons, addresses budget allocations, and outlines procedures for selection and de-selection of materials. (Contains a list of 22 resources.)