Meiofauna are ubiquitous members of aquatic ecosystems worldwide. Some of them can thrive under boundary conditions that are challenging to most other organisms. The discovery of well-adapted meiobenthic communities under extreme environments (e.g., sub-glacial sediments or deep in the Earth’s crust) has opened new insights into life under limiting conditions, insights relevant not only with respect to distributional and physiological aspects, but also of potential importance in research on venturous exobiological and medical frontiers. Although meiofauna do not have pelagic dispersal stages and, therefore, are directly exposed to the limiting environment of their immediate habitat, representatives from various taxa can survive environmental conditions often fatal to all other fauna. Typical ‘extremophiles’ belong to the abundant and diverse group of nematodes, but also, tardigrades, rotifers, loriciferans, kinorhynchs, and the protozoan foraminiferans (especially in polar regions). In addition, some can modulate their metabolism by temporarily switching to an ‘alternate life’ where they can ‘escape in time’ (Jönsson 2005). This extraordinary potential explains why they became preferred objects for biochemical, physiological, medical, and even exobiological studies. Natural extremes have always occurred, but under the present conditions of climate change, the frequency of extreme environmental challenges is increasing and represents a planetary threat. Therefore, understanding pertinent adaptations can provide vital information on how organisms may survive stressful challenges such as acidification, lack of oxygen, and rising temperatures. Thus, research on the biology, ecology, and physiology, including genetic studies, of these exotic meiobenthic taxa is gaining importance. As diversified as the taxonomic affiliations of extremophiles are their adaptive patterns that have evolved under hostile life conditions. The variety of pathways leading to their survival, including a temporary metabolic suspension or ‘dormancy’ (see Table 11.1), points to independent routes that have evolved repeatedly over evolutionary time. Pertinent studies may even shed light on their evolutionary diversification and their phylogenetic relationship (see Chap. 1). Since, in addition to nematodes, many specific studies are on tardigrades and rotifers, their life under extreme conditions, mostly in freshwater and desiccating terrestrial habitats, is also considered in this chapter.