The central theme of this book is host-parasite relationships with emphasis on comparative issues between different animal species. Where other contributors have highlighted similar systems, such as in fish, for a description of the interaction of avian species and their parasites there is a choice from a wide variety of parasite classes, families and species divided over all representatives of the avian kingdom. Part of the contents of this chapter was presented at the APS meeting on Comparative Physiology, San Diego, CA, August 2002 (Vermeulen et al., 2002).