Urea has long been of interest to both plant and animal physiologists. As early as 1889 De Vries demonstrated by plasmolytic methods that urea permeates plant cells. Since then it has played a large part in the development of theories of permeability, and more recently a non-osmotic uptake of urea has been postulated. The evidence for this is critically considered. The main pathway of urea assimilation is considered to be through enzymic hydrolysis by urease or in some instances perhaps by an enzyme complex. Specific products of a direct urea assimilation have been sought among the ureides. While these may be present only in very low concentrations in many higher plants, in others, for example the underground parts and the stems of Borraginaceae, Plantanaceae, Hippo-castanaceae, Aceraceae, and numerous leguminous plants, they account for up to half of the total nitrogen and are important N-storage substances. The seasonal and nutritional conditions which affect the formation of several of these ureides are considered. As a third possibility of urea assimilation, the possibility is considered of the direct formation of ureides, e.g., of allantoic acid. It has been proved that higher plants are capable of degrading purines and uric acid to allantoin, allantoic acid, glyoxylic acid, and urea, but the problem remains as to whether allantoin and allantoic acid are always formed via purines.