Based on research completed in 1977 and comments on a 1979 preliminary version of this report, this work evaluates current interlending practices among participants in the United Nations Educational, Scientific, and Cultural Organization (UNESCO), and proposes various models of interlibrary lending provision. The paper outlines the elements necessary in an interlending system, assesses the impact of technology on interlending, and presents factors relevant to the design and criteria for judging the effectiveness of interlending systems. Current practices are summarized as far as possible, and in the light of such information, potential basic and combined models are suggested. Examples of interlending systems draw mainly on those in the United States, United Kingdom, and Federal Republic of Germany, but a summary table covering 42 countries is also provided. Factors telling against efficient national interlending systems are enumerated, and the relationships between national interlending systems and other elements requiring national policy decision making are identified. A discussion of the mechanics of interlending is accompanied by a simplified flowchart of interlending procedures. On the basis of these findings, conclusions are drawn and further studies recommended. The report concludes with the reprint of a paper by Maurice Line and Stephen Vickers on the principles of national interlending systems. (Author/ESR)