[Greek]. Science, dealing with the flow and deformation properties of substances and materials. The flow characteristics and the deformation properties depend on the molecular structure of the substances and the interactions between the molecules. For Newtonian fluids like water the flow characteristics can be described via a linear time-independent correlation according to Newton’s law of viscosity. In view of the above, the viscosity of Newtonian fluids does not depend on the applied shear forces. For non-Newtonian fluids the viscosity changes depending on the applied shear forces and on the time of application of the shear force. If the viscosity decreases with increasing shear force, the effect is called shear thinning (structural viscosity). If the viscosity increases with increasing shear force, the phenomenon is called shear thickening (dilatancy). In both cases, the viscosity immediately returns to its original value after removing the shear force. If the viscosity increases at a constant shear force, the effect is called rheopexy. The opposite behaviour of decreasing viscosity at a constant shear force is called thixotropy. Rheopexy and thixotropy are time-depending effects. Due to this the viscosity does not return immediately to its original value after removing the shear force, but after a certain time of relaxation.