Fractals aspects in urban structures.— The introduction of fractals by B. Mandelbrot in the scientific discussion has given rise to a real boom of applications in all possible domains. In the present paper their application to urban structures will be discussed. The investigation of different aspects of the urban patterns, like the spatial distribution of the built-up areas or of different types of exploitation (supply systems, administration, etc.), as well as the dendrification of transportation networks leads to the more general point of view, that fractal dimensions are a useful quantitative mesure for all types of hierarchically organized subsystems. This can serve to compare those subsystems for different urban units. Finally, fractal growth processes, their simulation, and their growth dimension are introduced with respect to urban growth.