Electronic sensor armbands (ESA) appear to be valid and reliable in comparison to indirect calorimetry when measuring resting energy expenditure (EE). However, questions remain in terms of the ESA's ability to measure EE under exercise conditions. The purpose of this study was to evaluate the validity and reliability of the ESA to estimate the EE at rest and during selected activities of daily living. Fifteen volunteers, 10 females (29 ± 11 yrs.) and 5 males (40 ± 14 yrs.) were assessed on two occasions. On the initial visit, height and weight were recorded. Following 10 minutes of quiet rest heart rate and blood pressure were recorded. Participants then completed 20 minutes of level walking on a treadmill. The treadmill exercise was divided into two stages of walking; 10 minutes at 3 mph followed by 10 minutes at 4 mph. Following the treadmill exercise, subjects completed a six minute self paced walk test and then 10 minutes of self-paced stair exercise (walking up and down a 3 storey staircase). Exercises were separated by 5 minutes of quiet rest. Throughout the protocol, EE was measured by mobile open-circuit direct indirect calorimetry and the ESA. A mobile metabolic system (MMS) was used as the criterion measurement. Minute by minute oxygen uptake and respiratory exchange ratio (RER) was measured and EE (kcal·min−1) was calculated by multiplying the oxygen uptake (L·min−1) by the caloric equivalent based on the RER. In the second visit, subjects rested for 10 minutes and then completed the treadmill protocol that was used during the initial visit. At the initial visit the EE estimated from the ESA were significantly higher than that recorded by the MMS at both 3 mph (4.8 ± 1.3 vs. 3.8 ± 1.3 kcal·min−1) and 4 mph (6.8 ± 1.7 vs. 6.0 ± 1.4 kcal·min−1) p < .000. For the six minute walk test EE, ESA was 4.7 ± 1.8 kcal·min−1 vs. 3.5 ± 1.6 kcal·min−1 for the MMS p < .000. For the self paced stair exercise EE was; ESA (5.6 ± 1.6 kcal·min−1) and MMS (6.6 ± 3.3 kcal·min−1) p<.000. The second visit ESA EE at 3 mph was again significantly higher than that observed on the MMS (4.5 ± 1.6 kcal·min−1 vs. 4.1 ± .9 kcal·min−1 p<.001). EE measures between the ESA and MSS demonstrated an intraclass correlation of .745 and Cronbach's alpha of .854 p<.000 for the first visit and an intraclass correlation of .831 and Cronbach's alpha of .90 p<.000 for the second visit. The strong intraclass correlations suggest that the ESA was sensitive to changes in EE during treadmill walking. However, the ESA consistently overestimated EE for both self-paced and treadmill walking and underestimated EE during self-paced stair climbing. The use of ESA's to estimate EE during exercise is becoming increasingly popular and while these devices appear sensitive to variations in EE they have a tendency to overestimate EE. To improve the accuracy in these devices during activities such as self paced walking or stair climbing, subjects should be encouraged to swing their arms freely.