Recent epidemiological evidence suggests that prolonged sitting (sedentary behavior: time spent in behaviors that have very low energy expenditure, such as television viewing and desk-bound work) has deleterious cardiovascular and metabolic correlates, which are present even among adults who meet physical activity and health guidelines. Further advances in communication technology and other labor-saving innovations make it likely that the ubiquitous opportunities for sedentary behavior that currently exist will become even more prevalent in the future. We present evidence that sedentary behavior (too much sitting) is an important stand-alone component of the physical activity and health equation, particularly in relation to cardio-metabolic risk, and discuss whether it is now time to consider public health and clinical guidelines on reducing prolonged sitting time that are in addition to those promoting regular participation in physical activity.