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Measurement of real-world physical activity (PA) data using accelerometry in older adults is informative and clinically relevant, but not without challenges. This review appraises the reliability and validity of accelerometry-based PA measures of older adults collected in real-world conditions. Eight electronic databases were systematically searched, with 13 manuscripts included. Intraclass correlation coefficient (ICC) for <i>inter-rater reliability</i> were: walking duration (0.94 to 0.95), lying duration (0.98 to 0.99), sitting duration (0.78 to 0.99) and standing duration (0.98 to 0.99). ICCs for <i>relative reliability</i> ranged from 0.24 to 0.82 for step counts and 0.48 to 0.86 for active calories. <i>Absolute reliability</i> ranged from 5864 to 10,832 steps and for active calories from 289 to 597 kcal. ICCs for <i>responsiveness</i> for step count were 0.02 to 0.41, and for active calories 0.07 to 0.93. <i>Criterion validity</i> for step count ranged from 0.83 to 0.98. Percentage of agreement for walking ranged from 63.6% to 94.5%; for lying 35.6% to 100%, sitting 79.2% to 100%, and standing 38.6% to 96.1%. <i>Construct validity</i> between step count and criteria for moderate-to-vigorous PA was <i>r_s</i> = 0.68 and 0.72. Inter-rater reliability and criterion validity for walking, lying, sitting and standing duration are established. Criterion validity of step count is also established. Clinicians and researchers may use these measures with a limited degree of confidence. Further work is required to establish these properties and to extend the repertoire of PA measures beyond “volume” counts to include more nuanced outcomes such as intensity of movement and duration of postural transitions.