The incidence of overweight and obesity in the United States and worldwide has reached epidemic proportions. To effectively intervene, dietetics professionals and other health care practitioners need to address both sides of the energy balance equation when counseling clients and patients. Often, the focus on energy intake supersedes the promotion of a physically active lifestyle. Incorporating appropriate and sufficient physical activity into one's life is an essential component of achieving and maintaining a healthful body weight. This review summarizes background knowledge on the benefits of physical activity for health and provides an overview of available tools for measuring physical activity and energy expenditure. The physical and mental health benefits of an active lifestyle, current physical activity recommendations for the US public, the prevalence of inactivity in the United States, and components of energy expenditure are reviewed. Additionally, tools for estimating total energy expenditure, resting metabolic rate, and physical activity are evaluated and suitable approaches for applying these tools are provided.