With the introduction of new organic chemicals, the use of selective herbicides is finding broader application. Stimulated by war conditions, their use offers a promising method for increasing efficiency and reducing drudgery of agricultural production. Early work on selective herbicides is reviewed by ?slander (1), and by Bobbins, Crafts, and Raynor (17). This paper considers selectivity from a broader viewpoint, discussing the selective killing of annual weeds in crops of the family Umbelliferae by oils, the control of annuals and shallow-rooted perennials among deep-rooted perennial crops, and the selective use of growth regulators as sprays and soil amendments. Plants differ in many ways ; their surfaces vary in form and chemical composition ; some are easily wet by aqueous sprays ; others are difficult to wet. Surface coatings vary in permeability, and protoplasm responds specifically to different chemicals. The gross structure of plants varies in such a way that the meristematic tissues of some are exposed and vulnerable to sprays, others have their growing points protected. Because of these differences certain weeds may be destroyed in growing crops with little or no injury to the latter. The nature of selectivity