The selection of an ointment base in which drugs are to be incorporated for local application should depend on the condition of the patient's skin, the biological effect desired, and the pharmaceutical compatibility of the ingredients with each other and the base. The confusing array of official (U. S. P. and N. F.) and nonofficial bases which are available to the physician indicate the need for a survey of our knowledge of their pharmaceutical and biological properties. There are three major types of ointment bases: oleaginous (greasy) bases, which are water repellent; water-absorbing bases, which are greasy but allow absorption of water, and water-miscible bases (vanishing creams or water-soluble bases). Oleaginous or water-repellent greasy bases are the oldest group and contain representatives of animal, vegetable, and mineral origin. Vegetable oils are usually liquid at room temperature, but become solid when the unsaturated fats are hydrogenated.