To select a drug and dosage for a donkey, basic pharmacological principles have to be applied. There is the added disadvantage that few veterinary products include data on, or market authorisation for, the donkey. This chapter will briefly review some of the basic principles of pharmacology, outline some of the differences between donkeys and horses that may need to be taken into account in drug therapy, and provide background information on some of the main classes of drugs used commonly for donkeys.