Graying Baby Boomers and advances in medicine, technology and public health mean that by 2030 nearly one in five people in the U.S. will be 65 years old or older. The needs of this aging population will put unprecedented pressure on society, including on cities, through new demands on housing, transportation, public space, health care, and a wide range of services. This paper examines the role of cities in this demographic transformation by exploring the notion of elder-friendly communities, the relationship between human aging and the built environment, and a comparison of Age-friendly NYC in New York City and Lifelong Communities in Atlanta, two wide-ranging initiatives to make those places friendlier to older residents. It compares the two efforts to understand what strategies were developed to address the challenges unique to each place, and explores several major lessons that have emerged from which other cities might learn.