Precise measurement of sedentary behavior and physical activity is necessary to characterize the dose-response relationship between these variables and health outcomes. The most frequently used methods employ portable devices to measure mechanical or physiological parameters (eg, pedometers, heart rate monitors, accelerometers). There is considerable variability in the accuracy of total energy expenditure (TEE) estimates from these devices. This review examines the potential of measurement of ventilation ( ${\stackrel{\cdot }{\hbox{ V }}}_{\hbox{ E }}$ ) to provide an estimate of free-living TEE. The existence of a linear relationship between ${\stackrel{\cdot }{\hbox{ V }}}_{\hbox{ E }}$ and energy expenditure (EE) was demonstrated in the mid-20th century. However, few studies have investigated this parameter as an estimate of EE due to the cumbersome equipment required to measure ${\stackrel{\cdot }{\hbox{ V }}}_{\hbox{ E }}$ . Portable systems that measure ${\stackrel{\cdot }{\hbox{ V }}}_{\hbox{ E }}$ without the use of a mouthpiece have existed for about 20 years (respiratory inductive plethysmography). However, these devices are adapted for clinical monitoring and are too cumbersome to be used in conditions of daily life. Technological innovations of recent years (small electromagnetic coils glued on the chest/back) suggest that ${\stackrel{\cdot }{\hbox{ V }}}_{\hbox{ E }}$ could be estimated from variations in rib cage and abdominal distances. This method of TEE estimation is based on the development of individual/group calibration curves to predict the relationship between ventilation and oxygen consumption. The new method provides a reasonably accurate estimate of TEE in different free-living conditions such as sitting, standing, and walking. Further work is required to integrate these electromagnetic coils into a jacket or T-shirt to create a wearable device suitable for long-term use in free-living conditions.