Spatial patterns seem to be very stable in Los Angeles, the most volatile of all US metropolises. Processes suggested by the classical growth models to explain these patterns have ceased to be applicable over the last fifty years or so, and have probably never played any important role in Los Angeles. A new conceptual model is proposed to integrate the rapidity of urban change and the persistence of spatial patterns. Urban forms and their content should be considered as independent of each other. Urban change appears as a dialectical process, whereas the spatial order of a city seems to be a geometrical and topological phenomenon with a logic of its own. Urban forms persist because a city is a self-organizing system. Such systems, analyzed by biologists such as Atlan, are able to absorb random perturbation while conserving their order. This model is applied to some examples in the USA, Europe, and Algeria.
