Most publications devoted to diagnosis of helminths emphasize characteristic parasitological features. Histological feature are seldom described in detail. The purpose of this study was to propose a diagnostic method suitable for use by histologists who, unlike parasitologists, do not visualize the whole worm but rather pieces randomly scattered over a slide. It is relatively easy to distinguish helminths which have smooth muscle, no respiratory or circulatory system, and no coelom from arthropods which have striated muscle, both respiratory and circulatory systems, and coelom. At the adult stage, roundworms or nemathelminths present an external cuticle that may have patterned markings. The visceral cavity is empty. Adults can dwell either in the intestine in which case they are oviparous (oxyuris, ascaris, Ancylostoma, Strongyloides stercoralis, Trichuris...) or in tissue in which case they are viviparous (filaria). Larva of some species can be found in tissues. Adult tape-worms are devoid of cuticle and have a mesenchymatous visceral cavity. Cestodes which are segmented and have no digestive tract (taenias) are readily distinguishable from trematodes which are not segmented and have a digestive tract (faciolasis) Some cestodes unable thrive in man can cause cyst formation (hydatidosis, sparganosis, cysticercosis, coenurosis). On the basis of symptoms and histological features, it is usually possible to diagnose the genus and even the species of the offending helminth provided that the parasite has not been excessively damaged and that a sufficient number of sections are available.