The interday reliability of the Intelligent Device for Energy Expenditure and Activity (IDEEA) has not been studied to date. The study purpose was to examine the interday variability and reliability on two consecutive days collected with the IDEEA, as well as to predict the number of days needed to provide a reliable estimate of several movement (walking and climbing stairs) and nonmovement (lying, reclining, and sitting) behaviors and standing in older adults. The sample included 126 older adults (74 women) who wore the IDEEA for 48 hr. Results showed low variability between the 2 days, and the reliability was from moderate (intraclass coefficient correlation = .34) to high (.80) in most of movement and nonmovement behaviors analyzed. The Bland–Altman plots showed high–moderate agreement between days, and the Spearman–Brown formula estimated that 1.2 and 9.1 days of monitoring with the IDEEA are needed to achieve intraclass coefficient correlations ≥ .70 in older adults for sitting and climbing stairs, respectively.