The dynamic behavior and the stability of inhomogeneous fluid systems are mainly determined by rheological properties of internal and external interfaces. Special problems arise in the theoretical description of such systems due to the fact that an interface can be considered as a two-dimensional, highly compressible phase of variable composition, while the adjacent bulk phase is approximated as a three-dimensional incompressible homogeneous fluid. A mixed solution has different mole fractions within the bulk phase and within the interface which are determined in their equilibrium state by the laws of thermodynamics. A disturbance of the system by stretching or heating leads not only to a new tension state but also to relaxation processes. This means there exists a typical visco-elastic behavior as a result of the inhomogeneous structure of the fluid at the interface. For the description of the dynamic processes at the interface we need force balance equations and transport equations which take into account the anisotropic structure of the fluid in this area. The problem is the quantitative determination of the stress tensor and its divergence equations. A general theoretical calculation is impossible, however, experimental setups can provide information about interfacial rheological properties. Some new devices which measure surface dilational properties and surface shear properties improve the accuracy of results. This allows a detailed explanation of processes which are related with deformations of surfaces of surfactant solutions. Besides, a verification of adsorption hypotheses and the analysis of the tension state are possible. The correlation between foam stability and surface rheological properties can also be improved on the basis of these measurements and models. A molecular statistical model of the complex dynamics at a fluid interface is very difficult. Nevertheless, special hypotheses about the fluctuation of stress, momentum, and energy in subsystems can be used as basis for a statistical model with empirical parameters.