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Background: The aim of this study was to determine the relative and absolute reliabilities of 5 key performance-based measures of physical function in the Canadian Longitudinal Study on Aging (CLSA)., Methods: An age-stratified subsample of 147 participants from the CLSA who were undergoing their 3-year data collection visit participated in 2 repeat visits (within 1 week). Participants underwent tests of grip strength, 4-m gait speed, Timed Up and Go (TUG), chair rise, and single-leg stance (left, right, mean, maximum). Intraclass correlation coefficients (ICCs), standard error of measurement, and minimal detectable change (MDC) values were calculated., Results: The relative reliability for grip strength was excellent (ICC = 0.95); the TUG and single-leg stance tests had good reliability (ICC = 0.80 or 0.78-0.82, respectively); gait speed and the chair-rise test had moderate reliability (ICC = 0.64 for both) for participants overall. For participants between 50 and 64 years, TUG and gait speed had poor reliabilities (ICC = 0.38 or 0.33, respectively). For participants aged 75 years and older, the single-leg stance had poor reliability (ICC = 0.30-0.39). The MDC90 was about 6 kg for grip strength, 2.3 seconds for TUG, 0.2 m/second for gait speed, 5.2 seconds for chair rise, and ranged from 22.8 to 26.2 seconds for the single-leg stance., Conclusions: Among community-dwelling Canadians older than 50 years, the reliabilities of the CLSA measures were moderate to excellent. The TUG and gait speed in the youngest age group, and the single-leg stance in the oldest age group, showed poor reliability. MDC values can be used to interpret changes over time., (© The Author(s) 2021. Published by Oxford University Press on behalf of The Gerontological Society of America.)