Illegal physical development is an urbanization problem, so to discover the reasons behind the uncontrolled and illegal development; social, economic and political determinants such as industrialization, migration and urbanization processes should be analyzed. Therefore, to understand the process of illegal physical development, background of the phenomenon is discussed. Informal building samples from third world cities, from Turkish cities and from Izmir are given to present different variations for comparison. Policies, laws, regulations, applications and institutions involved in the prevention of illegal building production are evaluated. In the case study, the enforcement notices which were given by nine district municipalities of Izmir in year 2000 are analyzed to examine the municipalities'' acts and attitudes against the illegal-uncontrolled development. GIS is used in spatial analysis. Even though illegal/informal building process is mostly regarded as a housing problem, the findings of the case study prove that there are also industrial, commercial, tourism and other types of informal buildings.