The phenomenon of overtourism, characterized by its multifaceted impacts on destinations, has emerged as a major concern in the tourism industry. This dissertation aims to explore the dynamics of overtourism, emphasizing the dual impacts of main-source tourism markets on destinations in terms of their economic, social, and environmental consequences. Unlike existing literature, which focuses primarily on the negative aspects of overtourism, the present study illustrates the nuanced interaction between tourism markets by highlighting both their potential benefits and disadvantages. This study offers an in-depth analysis of cost and benefit factors based on a priori and a posteriori segmentation methodologies, combined with time-series analysis and limited information maximum likelihood (LIML) methods. Based on three case studies—Hong Kong, Malta, and Barbados—from 1980 to 2021, this study demonstrates the heterogeneous nature of the impacts across destinations and the complexities of market aggregation and interaction. The study identifies gaps in the conventional narrative of overtourism and introduces an interdisciplinary approach to the investigation. Based on the symbiotic framework, coupled with the Portfolio Theory, market aggregations and interactions can be classified into mutualism, commensalism, and parasitism. Additionally, the ‘limits of acceptable change' (LAC) and the ‘level of analysis problem' (LAP) frameworks have been utilized to further examine dominant and non-dominant markets' aggregation effects and interaction dynamics, resulting in a more comprehensive understanding of overtourism's complexity. Key findings suggest tailoring strategies to address overtourism, emphasizing the balance between minimizing costs and optimizing benefits. Based on the findings of this study, policymakers and stakeholders must develop strategies that respond to the challenges associated with overtourism by integrating empirical measures with theoretical frameworks.