"This document servers both as an introduction to CLU and as a language reference manual" CLU is a modular programming language. The development began in 1974 and the first version was finished in 1975. Following the design was reviewed and implemented, and the second version was designed in 1978. The first version of the manual appeared in 1978 and a second in 1979. This publication is contains an overview of the features of CLU and the differences from ordinary programming languages, which is followed the detailed discussion of the aspects and use of the features.