The Heligmonellidae is the most speciose family of the Heligmosomoidea, one of the three superfamilies of the Trichostrongylina, nematode parasites of the digestive tract of all groups of terrestrial vertebrates. The family comprises more than 360 species distributed worldwide, mainly parasitic in rodents. Their life-cycles being monoxenous, and their hosts relatively easy to breed in laboratory, these parasites are useful models for researchers addressing a variety of topics such as host-parasite relationships, biology, pathogenicity, and phylogeny. This book is a synthetic overview of our current knowledge of the Heligmonellidae family of nematode parasites.