These 20 species represent collections form the almost world wide distribution of this family. Anatomically all species studied were found to be homogeneous showing a gradual advancement from the most primitive species studied to the most advanced. From a morphological and geographical viewpoint three groups are recognized. The Clethraceae is generally considered to have but one genus, Clethra L. Results of this study show this to be a valid classification. The Clethraceae appears to be monophyletic in origin. Close relatives of the Clethraceae are thought to be the Theaceae and the lower Ericaceae. / Master of Science