Metabolic rate is a fundamental variable in thermal comfort research, as it determines the rate of internal heat production and directly affects human thermal sensation. This chapter reviews recent progress in the quantification of metabolic rate and its integration into outdoor thermal comfort evaluation. The literature indicates that while thermal indices such as PMV rely on fixed metabolic rate assumptions, real-world activity levels vary dynamically and contribute significantly to discrepancies in model predictions. Advances in interdisciplinary research, including physiological monitoring, wearable sensing, and metabolic modeling, have improved the ability to capture activity-related variations in comfort studies. Nevertheless, challenges remain in terms of accuracy, practicality, and the integration of metabolic rate with other human and environmental factors. Representing metabolic rate as a static parameter or considering it in isolation often leads to misinterpretation of field results. This chapter highlights the necessity of treating metabolic rate and activity levels as dynamic, context-dependent variables, and discusses how their systematic inclusion can improve the accuracy of thermal comfort indices and inform human-centered urban design.