As people age, they prefer to “age in place.” The concept of aging in place refers to the ability to live in one's own home, wherever that might be, for as long as one can feel confident and comfortable. Where people live and whether these environments can support them are critical questions for public health and public policy, especially since the baby boomers began to turn 65 on January 1, 2011. Equally important for public policy, those aged 85 and older are the fastest growing population group in the United States. The Health and Aging Policy Fellows Program, with the Centers for Disease Control and Prevention Healthy Aging Program, has supported a project to determine how design features of the built environment can support the mobility of older adults. Mobility refers to physical activity, usually walking, but also encompasses the ability to stay connected to nearby community resources and services. The project's purpose is to investigate features that support mobility in built environments. This policy brief introduces the realist synthesis method used in the project and selected national initiatives and activities to place this work in a broader context. Given the importance of mobility concerns to older adults, it must be determined without delay which design features support mobility and how local areas can better prepare to support the health of their aging populations.