OBJECTIVES: The present study sought to formulate a precise definition of sedentarism and to identify activities performed by active people that could serve as effective preventive goals. METHODS: A population-based sample of 919 residents of Geneva, Switzerland, aged 35 to 74 years, completed a 24-hour recall. Sedentary people were defined as those expending less than 10% of their daily energy in the performance of moderate- and high-intensity activities (at least 4 times the basal metabolism rate). RESULTS: The rates of sedentarism were 79.5% in men and 87.2% in women. Among sedentary and active men, average daily energy expenditures were 2600 kcal (95% confidence interval [CI] = 2552, 2648) and 3226 kcal (95% CI = 3110, 3346), respectively; the corresponding averages for women were 2092 kcal (95% CI = 2064, 2120) and 2356 kcal (95% CI = 2274, 2440). The main moderate- and high-intensity activities among active people were sports (tennis, gymnastics, skiing), walking, climbing stairs, gardening, and (for men only) occupational activities. CONCLUSIONS: The definition of sedentarism outlined in this article can be reproduced in other populations, allows comparisons across studies, and provides preventive guidelines in that the activities most frequently performed by active people are the ones most likely to be adopted by their sedentary peers.