Significance. There is growing concern for low levels of PA in all segments of the population, and increased interest in designing interventions. Physical Activity (PA) levels of children and youth have become a central focus in school-based research as well as other venues. This has prompted the development of numerous data collection tools to assess PA. Conceptual and Research Basis. Improvement in fitness levels of youth has been replaced by PA behavior as a key outcome measure, and is now prominently featured within national health objectives. Consequently, systematic observation has emerged as a central method for assessing of PA Systematic Observation has a long and rich history in Sport Pedagogy and countless other fields of study. Systematic observation is a powerful data collection tool, because it provides a direct measure of PA behavior, and also allows for assessment of variables of environmental conditions within which PA behavior occurs. Methods. This presentation will offer an overview of critical features of systematic observation including its advantages, disadvantages, observer training procedures, the various observation tactics (i.e., event recording, duration recording, interval recording and momentary time sampling), the use of coding conventions, and validity of PA codes. For example some advantages of systematic observation are that it exceeds other PA measures by being able to provide contextually-rich data on the environment, it has high internal validity, low inference, and low subject burden. Its disadvantages include personnel costs, need for observer training and recalibration, inaccessibility to certain environments, and potential subject reactivity. Information will be shared about how the various PA codes in established observation systems have been validated against several PA intensity measures, including heart rate, accelerometry, and O2 consumption. The same codes have been validated for use in assessing PA level s of pre-K through high school aged youth (incl. those with disabilities). As well, an overview will be provided on how systematic observation can be used for assessment of PA as both a process and product measure. Results and Conclusions. Attendees will develop a better understanding of the appropriate use of systematic observation as a method for assessing PA.Keyword(s): assessment, exercise/fitness/physical activity, research