The phylum Nematoda and the class Turbellaria contain a number of parasitic and free-living species, whilst the classes Monogenea, Trematoda, Cestoda and Acanthocephala contain wholly parasitic forms that occur in a range of vertebrate and invertebrate hosts. Each of these taxa are themselves further infected with a range of pathogenic and benign agents. This chapter aims to describe and list the range of infections in the Nematoda, platyhelminths and other parasitic metazoans across their aquatic and terrestrial range. The bulk of these infections are typically benign and consist of viruses, bacteria, protistans as well as metazoans. Further consideration is given to those infections considered to be detrimental to their invertebrate hosts and covers examples of hyperparasites, mutualism, and commensals where appropriate. However, although recognised, it does not cover details of those organisms that act as vectors for disease.