Sciences whose philosophical and theoretical bases are given by the general system theory, are developed by some kinds of paradigms. A basic paradigm of modern geography that is underlying the general system theory may be the spatial diffusion theory. Studies about the cities may be defined as the studies to demonstrate the parts of the urban systems.Urban system would be changed and generated by the diffusion of urbanization. It seems that the urbanization has four dimensions—urban growth, industrialization, emergence of middle class and the rise of the nationalism. The changes of those dimensions bring forth the changes of the social, economic, political and cultural systems which compose the urban system complex. However, those are nonspatial systems. Therefore, it needs that the nonspatial systems to project on earth-surface, and then they are transferred into the spatial systems as the models.The urban system complex can be studied by setting the resolution level. And the urban system theory can synthesize two traditional approaches for study of cities; the one is the study of city as the system and the other is the study of the systems of cities. Three cardinal resolution levels of the urban system are distinguished to urban geographers; daily urban system, regional urban system and national urban system. Above the national level, we can see the world urban system as the whole urban system. Although a city, It is composed of many subsystems and latent interactions. They are elements of a city system, and at the same time, they are elements of the regional and national urban systems. Those various levels of urban system compose the hierarchical urban system as the whole.On the viewpoints mentioned above, author would like to propose the present problems on the study of urban system, 1. The urban system should be analysed on the various resolution levels with many variables. On the basis of the outcomes of many analyses the urban system complex can be synthetically interpreted.2. It is important that the demonstration of the growth patterns of urban system in accordance with the progress of urbanization by the time-serial study. To study the growth patterns of the urban system could contribute to solve the actual urban problems and could provide useful advices for planning.3. The forcasting of the urban system in future is also important. It will be achieved by the synthetical study which includes the study of the urban system structure, shape, nature, boundary, dimension and the cause of changes along the time-serial axis. This may mean a study of the spatial diffusion effects of urbanization.4. Finally, the relationships between the urban system and regional system in the integrated space system should be systematized and the hierarchical structures should be made clear in their systems.