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The population of older adult women in the United States is diverse, ranging from individuals who are healthy and living independently to those who are homebound or institutionalized. Exercise and daily patterns of rest and activity may play an important role in the health outcomes of older persons, and the measurement of daily physical activity represents an area of growing interest in research on aging. Actigraphy has been widely used as a "field" instrument to monitor the frequency, duration, amplitude, and intensity of daily activity as older adults go about life in their typical environment. This article describes the kinds of variables and information that actigraphy can provide and explores measurement issues that need to be considered.