Fluids are called 'critical' or 'near-critical' when their temperature and pressure are near their gas-liquid critical point (CP). In a wide domain around the CP, important parameters (e.g. isothermal compressibility, density of gas and liquid phases, surface tension) obey universal power laws and are easily varied or scaled by using small changes in temperature. The highly variable properties of near-critical fluids make them very appealing for studying many interesting phenomena that are valid for all fluids. Above the critical temperature and pressure, such fluids are called 'supercritical' (Figure 8.1). In this region, fluids exhibit a number of specific properties (large density, low viscosity, large diffusivity) which make them intermediate between liquids and gases. In addition, their isothermal compressibility can become very large, especially when they approach the critical point. Their use under normal gravity conditions - or under reduced gravity, e.g. for the storage of cryogenic propellants - raises fundamental questions concerning fluid dynamics, heat transfer, interfacial phenomena and chemical process. Experimentation in the ISS is a good opportunity to answer these questions and enhance the knowledge in this field.