This background Note introduces the implications of climate change (global warming) for American cities. In the U.S., partisan political divides and unaddressed economic and racial disparities in climate vulnerabilities can inhibit action. The two main fronts for action are mitigation (reducing emissions) and adaptation (preparing for change by building resilience). The Note reviews main sources of emissions - energy, buildings, transportation, waste - and examples of mitigation actions, showing differential costs and impact. Adaptation actions depend on geographic location and specific climate hazards, with resilience of infrastructure and institutions a major factor. The most risk-prone are often the least prepared. Crises tend to propel action, and foundations have facilitated adaptation capacity, but leadership challenges require cross-sector goal-setting and collaboration.