What is a thriving city? Is it even possible to raise the well-being of an entire city, and why bother? Recent advancements in positive psychology have made it possible to define, measure, and increase well-being on a much larger scale. This provides an unprecedented opportunity for cities to explore well-being. In order to increase the well-being of the city – cities will need to think carefully about what that means, why it is important, and how they will do it. This capstone posits that cities can define what well-being means for themselves inclusive of: the target (the city, individuals, or other ecosystems, such as neighborhoods), the outcomes (the anticipated results of increased well-being), and the measures (how a city chooses to assess subjective and objective well-being). This capstone proposes that cities can utilize a positive psychology design thinking approach to define these outcomes and create optimal interventions to increase well-being at scale. Through literature review, case studies, and the introduction of a deliberate design thinking approach to applying and measuring well-being, this capstone provides an entry point for city leaders to begin understanding the science of positive psychology and practical application of well-being for their cities and citizens.