Segregation refers to nothing more than the “disproportionate distribution of population groups across urban sub-units” ^[1] (p. 217). It can be conceived very abstractly as the unequal distribution of “elements over subunits of a unit” (ibid.). “Elements” can be occupational or income groups, as well as religious groups, age groups, or people of a certain origin. “Sub-units” are usually residential areas of the “unit” city.

Issues of residential segregation have been studied on an international scale. They have been subject to longstanding political and theoretical debates and form an inter- and transdisciplinary field of study. Residential segregation includes various dimensions and domains, and a range of methods has been developed for measuring segregation.

The extent of the unequal distribution of the described characteristics can thus be expressed by different statistical measures. The segregation index ^[2], which measures the distribution of a population group in relation to the total population, is particularly frequently used in research. The index can take values between 0 and 100 and indicates the proportion of a population group that would have to move in order to achieve an equal distribution across the city. A value of 0 stands for a completely even distribution of the examined characteristic, and a value of 100 for complete spatial separation. However, great caution must be exercised when interpreting index values ^[3] since they depend strongly on the underlying division of the city as a whole into subareas. The larger the subareas, the more likely they are to contain different population groups and the lower the segregation index. In addition, there are different characteristics of urban areas (e.g., building density, physical conditions and building types). For this reason, the comparability of segregation indices between different cities is very limited.

Over the past 100 years, various approaches have been developed to explain segregation. Essentially, six currents can be distinguished.