Over the past few years, attention has been drawn to the importance of neighborhood influences on physical activity behavior and the need to consider a multilevel analysis involving not only individual-level variables but also social- and physical-environment variables at the neighborhood level in explaining individual differences in physical activity outcomes. This new paradigm raises a series of issues concerning systems of influence observed at different hierarchical levels (e.g., individuals, neighborhoods) and variables that can be defined at each level. This article reviews research literature and discusses substantive, operational, and statistical issues in studies involving multilevel influences on middle-aged and older adults' physical activity. To encourage multilevel research, the authors propose a model that focuses attention on multiple levels of influence and the interaction among variables characterizing individuals, among variables characterizing neighborhoods, and across both levels. They conclude that a multilevel perspective is needed to increase understanding of the multiple influences on physical activity.