It is impossible in a single volume to deal comprehensively with all classes of secondary plant compounds. In the earlier series of this Encyclopedia emphasis was laid on the isoprenoids and plant phenols. While these compounds have not been neglected in the present volume we have attempted to achieve a more balanced presentation by drawing attention to the importance of nitrogenous secondary metabolites such as the alkaloids, amines and non-protein amino acids. Most of the compounds or groups of compounds included in Volume 8 are of restricted distribution within the plant kingdom and wherever possible we have provided information concerning their chemistry, biochemistry, taxonomic signifi­ cance and probable ecological roles. Secondary compounds cannot be defined in terms of restricted distribution, however, nor can they be defined without refer­ ence to the plants in which they occur, as it is possible that a given compound occurring in two species may have a primary role in one and not in the other. As our knowledge of biochemistry increases we shall no doubt find it necessary to revise our ideas concerning the roles of a great many of the compounds which are found in plants.