Obsolescence deals with changes in the use of literature with time. This article describes the origin and development of obsolescence study. It also examines the assumptions, methodologies and conclusions of significant researches on the topic, especially, lhe distinction between synchronous and diachronous studies. In addition, the reasons for decline of use overtime, the applications and limitations of obsolescence research results and factors that affect obsolescence are discussed. Moreover, an experimental research to explore the relationship between the journal citation half-life and library use half-life is demonstrated.
