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Measuring physical activity with objective tools, such as accelerometers, is becoming more common. Accelerometers measure acceleration multiple times within a given frequency and summarize this as a count over a pre-specified time period or epoch. The resultant count represents acceleration over the epoch length. Accelerometers eliminate biases associated with self-reporting measures of physical activity. With the increasingly widespread use of accelerometers, standardization of how the data are collected and reported across studies is needed. In 2005, Masse and colleagues identified five methodological issues regarding reducing accelerometer data to derive summary measures: (1) identifying accelerometer wearing time; (2) defining minimal wear time for a valid day; (3) identifying spurious data; (4) computing summary variables and aggregating days of data; and (5) extracting bouts of activity. This study focuses on the first issue; specifically, identifying nonwearing time of the accelerometer. Understanding whether the accelerometer is worn helps researchers assess compliance and determine if the participants' data will contribute to the resulting analyses. Defining wearing and nonwearing time also affects the derivation of summary physical activity measures, such as average counts per hour worn or minutes in sedentary, light, moderate, or vigorous activity. (Contains 2 tables and 1 figure.)