Ordered layers and structured packings are used as internals in numerous rectification columns. They characteristically exhibit low pressure drops and high plate numbers per unit length. The relatively high investment costs are frequently offset by small column dimensions and hence lower equipment costs. In packings of industrial dimensions unstable flow states may occur which significantly lower the plate numbers. Such instabilities can be detected experimentally by tomographic methods. Moreover, flow patterns can be predicted theoretically. A theoretical description permits the engineer to scale up such columns containing packings and to deduce the design features necessary for industrial scale columns from laboratory experiments.