Helminths are worm-like parasites that utilise a variety of vertebrates and invertebrates as hosts. They include various phyla such as Platyhelminthes (flatworms), Acanthocephala (thorny-headed worms) and Nematoda (roundworms). The Platyhelminthes include the Monogenea, Cestoda and Trematoda. Although some species of these phyla are free-living (e.g. some flatworms such as Turbellaria and several taxa within the Nematoda), the species with parasitic lifestyles are very diverse and exhibit a wide range of life cycles and transmission strategies. They are found in almost all freshwater and marine systems, and many of them play an important role and have a significant impact on ecological interactions and ecosystem health. Often, these parasites can also bridge between different ecosystems, that is, between terrestrial and aquatic systems, due to the variety and diversity of hosts they utilise. In this chapter, we cover the biological underpinnings of these parasite groups, including their life cycles, morphology and impacts on their hosts. This forms a valuable basis and reference for the helminth examples given in the following chapters.