Research indicates that high levels of sedentary behavior(sitting or lying with low energy expenditure) are adversely associated with health. A key factor in improving our understanding of the impact of sedentary behavior(and patterns of sedentary time accumulation) on health is the use of objective measurement tools that collect date and time-stamped activity information. One such tool is the activP AL monitor. This thigh-worn device uses accelerometer-derived information about thigh position to determine the start and end of each period spent sitting/lying, standing, and stepping, as well as stepping speed, step counts, and postural transitions. The activP AL is increasingly being used within field-based research for its ability to measure sitting/lying via posture. We summarise key issues to consider when using the activP AL in physical activity and sedentary behavior field-based research with adult populations. It is intended that the findings and discussion points be informative for researchers who are currently using activP AL monitors or are intending to use them. Pre-data collection decisions, monitor preparation and distribution, data collection considerations, and manual and automated data processing possibilities are presented using examples from current literature and experiences from 2 research groups from the UK and Australia. Research indicates that high levels of sedentary behavior (sitting or lying with low energy expenditure) are adversely associated with health. A key factor in improving our understanding of the impact of sedentary behavior (and patterns of sedentary time accumulation) on health is the use of objective measurement tools that collect date and time-stamped activity information. One such tool is the activP AL monitor. This thigh-worn device uses accelerometer-derived information about thigh position to determine the start and end of each period spent sitting / lying, standing , and stepping, as well as stepping speed, step counts, and postural transitions. The activP AL is increasingly being used within field-based research for its ability to measure sitting / lying via posture. We summarize key issues to consider when using the activP AL in physical activity and sedentary behavior field-based research with adult populations. It is intended that the findings and discussion points be informative f or researchers who are currently using activP AL monitors or are intending to use them. Pre-data collection decisions, monitor preparation and distribution, data collection considerations, and manual and automated data processing possibilities are presented using examples from current literature and experiences from 2 research groups from the UK and Australia.