The natural communities of the world are diverse, and many schools of ecology have developed classifications of communities in partial independence of one another. There is consequently a vast and widely dispersed literature on the classification of plant and animal communities, comprising divergent approaches of different schools and representing a great experiment on the usefulness of different possibilities for classification. The editor sought in a re­ view monograph of 1962 to summarize these schools and their history, and in 1973 published a treatise on 'Ordination and Clas­ sification of Communities' as volume 5 of the Handbook of Vegetation Science. We were fortunate, in preparing the latter work, to have a truly international panel of authors to discuss different major ap­ proaches to classification. This second edition of the book of 1973 is intended to make the work more widely available in a less expensive form as companion volumes on ordination and on classification of plant communities.