This book provides a coherent account of the Theory of Classification. It discusses the contribution made by theoreticians like E.C. Richardson, J.D. Brown, W. Hulum, W.C. Berwick Sayers, H.E. Bliss and S.R. Ranganathan. However, the theory put forward by S.R. Ranganathan predominates the whole book because his contribution is far more than anybody else???s. Five major schemes of Classification, Library of Congress Classification, Colon Classification and Bliss Biblio-Graphic Classification, have also been discussed.