Downtown revitalization has become an important policy issue, especially in advanced countries. In the more than 50 years since Jane Jacobs’s critique, the mechanism of the formation and decline of downtowns has been the subject of much controversy. It has been argued that the agglomeration and attractiveness of downtown are supported by the internal diversity of downtown. However, because many controversies have remained in the discourse, few metrical analyses using operational system models based on survey data have been attempted. In this book, the authors identify the principle of the dynamics of downtown through microscopic re-interpretation of existing macroscopic spatial interaction models and meso-scale model construction from the microscopic point of view. Focusing especially on shop-around behaviors of downtown visitors as the key concept, the authors address (1) a rich trove of facts based on findings from a series of surveys conducted over more than a decade, (2) a review of existing shop-around behavior models and an exploration of a Downtown Dynamics model through gaming simulation, and (3) construction of a “boundedly-rational but intelligent” visitor agent model, development of ASSA (agent simulator of shop-around) visitors by such an intelligent agent-based approach, and implications of its simulation analyses. The book describes a research program to explore the mechanistic principle of Downtown Dynamics, especially the role of diversity that brings “co-evolution” both to visitors and to shop configuration in downtown areas. Included is a novel research program derived from complexity system science that provides approaches to researchers and graduates of spatial economy, regional sciences, geo-informatics, and urban planning studies as well as to planning officials and practitioners such as town managers and planners who are concerned with downtown revitalization.