Habitats with extreme conditions, such as the deep sea or polar regions, were long considered ‘retreats’ where organisms could escape and evade their parasites. However, parasitic organisms occur in literally every habitat on Earth, and many aquatic parasites have successfully adapted their complex and intricate lifestyles to extreme conditions, from ice-covered arctic regions to the scathing temperatures of deep-sea vents. In this chapter, we illustrate the distribution and specialised adaptations of metazoan parasites to different naturally occurring extreme conditions in aquatic environments. Although few of the metazoan hosts and parasites exist at the extreme chemical and physical boundaries of life under which only truly extremophile microbial organisms can survive, parasitic organisms have evolved to co-exist with their hosts in aquatic habitats that pose unique challenges and conditions for their lifestyles. Parasites in extreme environments constitute diverse, highly specialised, and ecologically important components of these unique ecosystems. Moreover, studying parasites under naturally occurring extreme conditions can help us better understand how host-parasite systems might respond to environmental changes and newly developing anthropogenic extremes.