Sedentary behavior (time spent sitting during waking hours) is recognized as a distinct behavior from physical activity with its own health effects and cannot be solely defined by lack of physical activity. Sedentary behavior is associated with increased risk of premature deaths and morbidity (type 2 diabetes, obesity, cardiovascular diseases, cancers, depression, dementia, etc.). The effects of physical activity are often insufficient to counteract the consequences of high sedentary time, especially if the level of physical activity is low. The mechanisms linking sedentary behavior to disease pathology are numerous and complex. The value of the "harmful" threshold for sedentary behavior in health is not defined, as it depends on all 24-hour physical activity behaviors (including sleep, standing, light, moderate, or vigorous physical activity), Three types of actions are effective in combating the effects of sedentary behavior: (1) substituting sedentary time with physical activity, meaning replacing a portion (30 minutes to 1 hour) of sedentary time with physical activity of any intensity; (2) regularly interrupting sitting time with active breaks; (3) engaging in light-intensity physical activity during sitting time (active desks). Given the significant amount of sedentary time (7 to 12 hours/day), reducing sedentary behaviors is a major public health goal and should be accompanied by messages promoting physical activity. (c) 2025 l'Academie nationale de medecine. Published by Elsevier Masson SAS. All rights are reserved, including those for text and data mining, AI training, and similar technologies.