This paper attempts to address the significant issues of library inter-dependence in a resource sharing environment. The concept of cooperative collection development has emerged as a vital component in an individual library's efforts to supply the information needs of its patrons. This is discussed in the context of both national and local requirements. Specifically, attention is focused on (1) the idea of cooperation today, (2) its antecedents, (3) the forces promoting cooperative collection development, (4) obstacles to its accomplishment, (5) the role of collection policy statements, and (6) a case study of resource sharing at the local level, utilizing the Colorado experience. This is an edited version of a paper delivered at the RTSD Collection Management and Development Institute, Irvine, CA, September 4-7, 1984.