Osmoregulation is the regulated control of water and solute composition in body fluid compartments. On one hand, the internal composition must be kept within optimal conditions for metabolic processes in the face of external perturbation. On the other hand, the nature of the living state demands a continuous traffic of compounds in and out of the organism. These demands appear to be in fundamental contradiction however cells and animals achieve so-called “steady-state” by means of an array of transport proteins, which provide a stringent control on the exchange of water and solutes across body surfaces. The distinct mechanisms of solute transport have been studied in most animal groups, but there are still large gaps in our understanding of how animals cope with osmotic stress. In the present thesis, osmoregulatory phenomena were studied in vertebrate and invertebrate organism alike, with the main focus being on fluid and solute dynamics in Tardigrada. For example, the inorganic ion composition of several species was investigated, which revealed that tardigrades contain roughly similar relative contributions of inorganic ions to total osmotic concentration, when compared to closely related animal groups. Moreover, it was inferred that cryptobiotic tardigrades (species able to enter a state of latent life) contain a large fraction of organic osmolytes. The mechanisms of organic anion transport in a marine species of tardigrade was investigated pharmacologically, and compared to that of insects. These data showed that organic anion transport is localized to the midgut epithelium and that the transport is both active and transporter mediated with a pharmacological profile similar to that of insects. Tardigrades survive in a variety of osmotic environments (semi-terrestrial, limnic and marine habitats), why the ability to volume and osmoregulate was examined. These studies demonstrated an ability to regulate total body volume during both hypo- and hyperosmotic conditions, and that the ability to hyper-regulate could be a general theme among members of eutardigrades. Thus, the work presented herein, have contributed to establishing tardigrades as an important experimental group in which central physiological questions may be answered, including aspects of osmotic and ionic regulation.