This chapter presents the practical applications of hydroxypyrimide fungicides, and their chemical, physical, and toxicological properties. Although there may be in vitro effects of these hydroxypyrimides on some fungal pathogens, in practical terms use is restricted to control of members of the Erysiphales. Thus, hydroxypyrimides are well suited to integrated control systems. The extreme specificity of hydroxypyrimidines toward powdery mildews limits their wider use, but adenosine deaminase is present as a potential target site in many other fungi. Availability of hydroxypyrimides in soils depends very much on moisture, acidity, and organic matter content, and strong adsorption of ethirimol on acid peat prevents use in these soils. Some indication of the likely biochemical mode of action of hydroxypyrimide fungicides arose through attempts to reduce their toxicity with various metabolites. Hydroxypyrimides are less mobile in woody plants, although bupirimate seems sufficiently volatile to be redistributed as vapor across leaf surface.