Abstract One of the first systems developed for hyperfiltration in the 1960's was a thin-channel system. In the 1970's, DDS developed a thin-channel system with short channels which proved to be excellent for hyperfiltration as well as for ultrafiltration. A description of the system is given. Examples of the applications of thin-channel systems for concentration at high viscosities and high dry-substance contents, e.g. in the food industries, are given. The theory of thin-channel systems is briefly described, especially in respect to answering to question why thin-channel systems are especially suited for treatment of highly viscous liquids. Concentration polarization is an important factor in determining capacity and membrane-separation properties. The concentration polarization can be calculated theoretically, but is difficult to measure quantitatively. A description of a thin-channel system is given, in which the membrane surface can be seen visually. In such a system, the concentration polarization can be shown visually, using coloured solutions. In this way, the influence of flow and pressure on the concentration polarization can be shown by photographs of the concentration-polarization profiles. A survey of the thin-channel systems on the market today is given.