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...