TROST, SG, KL MCIVER, and RR PATE. Conducting Accelerometer-Based Activity Assessments in Field-Based Research. Med. Sci. Sports Exerc., Vol. 37, No. 11 (Suppl), pp. S531–S543, 2005. The purpose of this review is to address important methodological issues related to conducting accelerometer-based assessments of physical activity in free-living individuals. We review the extant scientific literature for empirical information related to the following issues: product selection, number of accelerometers needed, placement of accelerometers, epoch length, and days of monitoring required to estimate habitual physical activity. We also discuss the various options related to distributing and collecting monitors and strategies to enhance compliance with the monitoring protocol. No definitive evidence exists currently to indicate that one make and model of accelerometer is more valid and reliable than another. Selection of accelerometer therefore remains primarily an issue of practicality, technical support, and comparability with other studies. Studies employing multiple accelerometers to estimate energy expenditure report only marginal improvements in explanatory power. Accelerometers are best placed on hip or the lower back. Although the issue of epoch length has not been studied in adults, the use of count cut points based on 1-min time intervals maybe inappropriate in children and may result in underestimation of physical activity. Among adults, 3–5 d of monitoring is required to reliably estimate habitual physical activity. Among children and adolescents, the number of monitoring days required ranges from 4 to 9 d, making it difficult to draw a definitive conclusion for this population. Face-to-face distribution and collection of accelerometers is probably the best option in field-based research, but delivery and return by express carrier or registered mail is a viable option. Accelerometer-based activity assessments requires careful planning and the use of appropriate strategies to increase compliance. Key Words: PHYSICAL ACTIVITY, OBJECTIVE MONITORING, ASSESSMENT, EXERCISE, VALIDITY, RELIABILITY