This publication contains the Review Lectures given at a joint NATO Advanced Study Institute and a FEB S Advanced Study Course, held at Sogesta (Nr. Urbino), Italy from the 7th - 18th May 1979. The Course entitled Nucleoside Analogues : Chemistry, Biology and Medical Applications was held for several reasons. In the past few years, many useful and potentially-useful nucleoside analogues have either reached the stage of clinical use or are undergoing clinical trials. Many more compounds have been synthesised by the organic chemist and little more has been done with them other than possibly a few perfunctory biological tests. This is often due to either a lack of interest or an inadequate knowledge of the testing proced ures available or a lack of communication between the chemist, biochemist, pharmacologist and the clinician such that few compounds receive the testing and evaluation which they deserve. The aim of this meeting was to gather together many of the experts in the different scientific disciplines which are involved in the design, synthesis, testing and clinical use of nucleoside analogues, primarily as anti-viral and anti-cancer agents, and to discuss in depth the fundamental principles of each discipline so that participants could understand each other's problems and be more aware of the information required and that which can be obtained.