The discussion on dynamics is related to the issue of equilibrium. Equilibrium models are based on the assumption that interdependent model variables, such as prices, supply and demand, adjust to equilibrium with zero delay or, if adjustment is delayed, equilibrium is eventually reached. Dynamic models, on the other hand, are based on the assumption that some changes, e.g. changes in demand, are faster than others, e.g. responses of supply, and that these differences in speed of adjustment are so large that urban systems are normally in disequilibrium. All but three (BOYCE, IMREL, KIM) of the twenty models are recursive simulation models. Recursive simulation models are called quasi-dynamic because, although they model the development of a city over time, within one simulation period they are in fact crosssectional. This is however only true for strictly unified models. Composite models consist of several interlinked submodels that are processed sequentially or iteratively once or several times during a simulation period. This makes composite models well suited for taking account of time lags or delays due to the complex superposition of slow and fast processes of urban development (cf. Wegener et al., 1986). However, this feature is insufficiently used by some models, because their simulation period of five years has the effect of an implicit time lag – a too long time lag in most cases. This problem is likely to disappear as faster computers will make shorter simulation periods of one or two years more feasible. ‎[19]