Parasiticides have been the most valuable weapons to combat parasites for almost half a century, constituting more than 50 % of veterinary pharmaceuticals and a good part of the products destined for human health. However, in many parts of the world, parasites develop resistance to these drugs, and it is now a major health problem. One way to avoid this inconvenience is the in-depth knowledge of its mechanisms of action, which will allow a more appropriate use. This work is a review of the different physiological aspects of parasite species and an explanation of the different mechanisms of action of antiparasitic agents. Once these drugs are ingested or absorbed by parasites the agents come into contact with the parasites structures, and according to the drug's properties it will define the antiparasitic activity either by altering tegument, carbohydrates, protein and lipid metabolism, or motility of the multicellular parasites.Facultad de Ciencias VeterinariasConsejo Nacional de Investigaciones Científicas y Técnicas