This dissertation investigates the transitions of urban design models in practice: the ways in which practitioners have adopted the urban design models and the factors that have influenced such adoption. In particular, this dissertation focused on the unexpected consequences of the adaptations of urban design models and distinguished these effects from those stemming from the inherent limitations of urban design models themselves. The major goal of this dissertation is to clarify the patterns of the transitions associated with urban design models in practice (particularly adaptation) to ensure a better understanding their impact on the urban environment. However, the transitions of urban design models in practice are complex phenomena that multiple actors with diverse interests have participated in and implemented numerous principles of the models over a long period of time and in diverse contexts. Therefore, to minimize such complexities while capturing important elements of the diffusion and adaptation processes, this dissertation presented a theoretical framework, the Model-Prototype-Adaptation (MPA) framework, based on recurring patterns of urban design movements. In the MPA framework, a "model" refers to an integrated set of urban design principles derived from a consensus of opinion of the enthusiastic proponents of an urban design movement; "prototypes" are projects developed by enthusiastic proponents who have strong commitment to the model and the movement; and "adaptations" are projects developed by eclectic followers who have weak commitment to the model and the movement and take advantage of the model for their interests and concerns. With these three key elements, the MPA framework hypothesizes two distinct transitions of urban design models in practice: "evolution," the developmental transition from old prototypes to new prototypes by enthusiastic proponents seeking to more effectively embody the model; and "divergence," a "watered down" application of the model in practice by eclectic followers responding to external factors such as market forces. This dissertation fleshed out the proposed basic MPA framework with historical reviews of the three urban design movements (Garden City, City Beautiful, and Modern) and a literature review of innovation-diffusion theories. In particular, the literature review focused on theories that present major factors influencing the adoption of innovations. The theories suggested that the ways in which adopters, who have different innovativeness and roles, perceive the attributes of innovations influence their decisions to adopt the innovations. In addition to the theoretical construction of the MPA framework, this dissertation presented a comparative case study with New Urbanist practices to test the MPA framework in a real world context. In particular, "divergence" of New Urbanism principles was examined specifically through a comparison of the six matched prototype-adaptation pairs of neighborhood developments in the Atlanta area. The case study first hypothesized three predictions about the perceptions and implementation of New Urbanism principles based on the MPA framework, that is, 1) enthusiastic proponents of New Urbanism perceive New Urbanism principles more positively than eclectic followers; 2) prototypes developed by enthusiastic proponents incorporate more New Urbanism principles and do so more thoroughly than adaptations developed by eclectic followers; and 3) New Urbanism principles that actors perceive more positively are implemented more often and more thoroughly. Data for the case study have been collected through interviews, surveys, field observations, planning documents, and local periodicals. The methods of analysis that were used in this study were pattern matching between predictions and observations, the explanation-building for the findings from pattern matching based on detailed contextual information derived from each case, and finally, cross-case synthesis. The comparative analysis showed that the case observations generally confirmed the three predictions. For example, among the New Urbanism principles, the "creation of an identifiable neighborhood" was perceived the most positively and also implemented the most often and thoroughly by both the enthusiastic proponents and the eclectic followers while "access to public transit" was perceived the least positively and implemented least often by both groups. In addition to the general confirmation of the three predictions, the analysis also revealed numerous unexpected findings, and efforts to build explanations for such findings based on the detailed contexts of each case yielded several important insights: the issue of compatibility between the thorough implementation of the New Urbanism model and the supply of affordable housing; the possibility of positive externalities from the proximity of prototypes to adaptations; two distinct flexibility arguments--flexibility for incremental accomplishment and that for contexts; the extent of public-private partnerships that broaden the influence of the New Urbanism principles beyond project boundaries; and communication problems between enthusiastic proponents and eclectic followers.