The deliberate art of city-state design is evident in Roman, Egyptian and Mesopotamian ruins. Driven in ancient times by power and defense, urban planning evolved as a recognized profession in the 18th century, scaled to displace permeable green spaces with housing, utilities, hardscape, and transportation venues, and to manage urban waste as a prophylactic against contagious disease. By the 20th century, urban planning uniformly adopted the Haussmann model, a hierarchical plan strategy based on zoning schemes that compartmentalized or segregated different land uses (Coburn, J. 2004). Without intent, fanfare or restraint, this model launched and institutionalized the framework for urban heat islands, city geometries that squander natural resources, tax built systems and challenge social equality, often at the cost of compelling public health considerations. The human, social and environmental costs of chronic urban heat stress follow twin trajectories: a quantitative one of increasing population and demand, and a qualitative one where the most vulnerable members of an urban ecosystem risk and suffer the greatest. To temper the impacts and consequences of heat islands, an ascendant planning model has emerged that uses urban forests to recapture resources, reconnect society, and restore public health. The road to cool, however, is a destination that can only be reached through the collective skill, boldness and imagination of planners, engineers, scientists, ecologists, urban foresters, public leaders, and others who envision the 21st Century City as an ecosystem built from gray, green and blue infrastructure, and people.