So much information concerning alkaloids has been published since the third edition of this book was issued in 1939, that the preparation of a new edition has involved re-writing a large part of the volume and adding considerably to its bulk. The material available has been dealt with, as in the previous edition, primarily on the. basis of a chemical classification according to nuclear structure, but as Nature does not produce alkaloids to meet the needs of either botanical or chemical systematists, strict observance of such a system would in some measure obscure those biological relationships among alkaloids which are at present attracting much attention from research workers. Accordingly this primary chemical classification has been modified, in cases where an extensive series, including several chemical types, occurs in one plant, or in closely related plants, when this results in a chemical group being dealt with in more than one place, cross references have been provided as a convenience to the reader.