This study examined the reliability and validity of the SenseWear Armband (SWA, BodyMedia, Inc.) during rest and exercise compared to indirect calorimetry (IC). Energy expenditure (EE) was assessed with SWA and IC in 13 males during two resting and one cycle ergometry (40 min at 60% VO_2peak) sessions. In a second experiment, 20 adults walked on a treadmill for 30 min at 3 intensities while IC and SWA measured EE. At rest, no significant differences were found between EE measurements from the SWA (1.3 ± 0.1 kcal/min) and IC (1.3 ± 0.1 kcal/min), and the methods were significantly correlated (r = 0.76). The SWA EE estimation was reliable when comparing the two resting visits (r = 0.93). For the ergometer protocol, no significant differences were found between the SWA and IC measurements of EE early, mid, or late in exercise or for the total bout, although the measurements were not correlated (r = 0.03-0.12). The SWA EE estimate of walking increased with treadmill speed but not with inclination. The SWA significantly overestimated the EE of walking with no grade (27.4% for 3mph; 12.6% for 4mph) and significantly underestimated EE on the 5% grade (21.9%) (p<0.02). The SWA estimation of EE correlated with IC (r = 0.47-0.69). The SWA provided valid and reliable estimates of EE at rest. The SWA provided similar mean estimates of EE as IC on the ergometer, however the individual error was large. The SWA overestimated the EE of flat walking and underestimated inclined walking EE.