Most internal parasites of vertebrates require stages outside the host for development and transmission. Some life cycles are simple and straightforward. Others may have one or more intermediate or paritenic hosts. Knowledge of life cycles of parasites first of all is of great scientific interest. Secondly, life cycles are of great importance in controlling parasites. The object of this presentation is to review life cycles of some mammalian parasitic nematode species in research in association with the University of Kentucky.