In the vast field of medical sciences, parasitology constitutes a small segment and has received comparatively scant attention, though worm infestation is a medical and public health problem of great magnitude, both in man and domesticated animals. The economic loss in terms of morbidity in man, loss of man-hours of work, and poor quality of animals, is exorbitant. Increasing the areas under cultivation, and quick and extensive modern transport system has contributed to the spread of worm infection beyond its traditional tropical and sub-tropical habitat. Drugs used as anthelmintics prior to World War II were mostly developed on empirical basis. Advances in the knowledge of physiology and biochemistry of parasites in last few years, have helped to a certain extent, in understanding the mechanism of action of known drugs, and development of newer chemotherapeutic agents. Understanding the mechanisms of action of anthelmintics, requires a more complete understanding of helminth physiology, biochemistry, effect of environment, and role played by immunological processes in parasitic diseases, than is available at present.