Fluid materials, by definition, are systems which flow when subjected to stress. How they respond to an input stress is the heart of rheological testing and may be a complex issue. There are many types of fluids: pure substances, mixtures, dispersions and solutions, falling into the categories of either simple or structured fluids. Each has its own unique behavior when subjected to stress. In general, when a material has a uniform phase, such as a solution or pure substance, it is referred to as a simple fluid. Materials which contain more than one phase, such as solid particles dispersed in a liquid, gas particles in foam or an emulsion of immiscible liquids are considered structured fluids since their rheological behavior is in general dominated by the interactions of the constituents.