Perhaps the two most important demographic shifts in the world today are urbanization and population aging. More than half the world’s population is now living in cities, and there are more people over the age of 65 than under the age of 5 worldwide. This is an enormous demographic shift and will lead to a world greatly changed from that of a mere 50 years ago. Aging populations in cities represent a tremendous opportunity to harness human capital on a previously unimaginable scale, but, conversely, may lead to challenges to ensure that services are available to meet the needs and wants of these populations. The health of aging populations is rapidly emerging as one of the central urban health issues of the coming decades. This chapter discusses changing age demographics in cities worldwide and accompanying challenges and opportunities for urban health.