Numerous regional economic models have been devised in the past four decades, motivated by the development of regional science and the desire to obtain an understanding of the working of regional and subregional economic systems. In addition to this academic interest, an important driving force underlying their development has been the needs of national and subnational economic planning, in order to help assess the impact of national economic trends and policies upon particular regions, and to illuminate the structure of local economies. Regional economic models have been of three basic types, becoming more sophisticated over time.