Significance. Ecological and cognitive-behavioral approaches to understanding physical activity (PA) strongly support the assessment of the physical and social environments in which the activity occurs. Systematic observation exceeds other measures of PA by providing contextually-rich data on the environment. Conceptual and Research Basis. There is growing concern for low levels of PA in all segments of the population, and increased interest in designing interventions. Meanwhile, PA occurs in specific environments which lend themselves to study and remediation, but researchers are often at a loss for usable and validated tools. Methods. This presentation compares and contrasts the methods, strengths, limitations, data output, and utility of four published systems for assessing PA and associated contextual variables. They include SOFIT (System for Observing Fitness Instruction Time), the most popular of the systems, has been used in the study of physical education classes in over 40 refereed publications. Time sampling tactics are used to assess students' PA, concurrent lesson contexts, and instructor behaviors. SOPLAY (System for Observing Play and Leisure Activity in Youth) is also used in school environments. However, it targets PA in leisure time settings (e.g., recess and other before, during, and after school programs). In addition to the numbers of students participating and their activity levels, SOPLAY measures activity area characteristics (e.g., temperature; accessibility; supervised; equipped, structured). SOPARC (System for Observing Physical Activity and Recreation in Communities) expands upon SOPLAY to study leisure time PA in parks, playgrounds, and recreation centers. It also adds procedures for simultaneously coding of gender, ethnicity/race, and age groupings. BEACHES (Behaviors of Eating and Activity for Children's Health: Evaluation System) was designed specifically for particularly home settings, and provides for the coding of both physical and social environmental conditions (e.g., child location; presence of parents, siblings, and peers; prompts and consequences for PA; media viewing; ingesting food). While all four systems are based on the same PA codes, each is unique in its assessment of the context in which PA occurs. These systems have been integral parts of longitudinal studies supported by the National Institutes of Health. Results and Conclusions. Four validated systems with similar activity codes are available for assessing PA and estimated energy expenditure in diverse settings. Researchers interested in understanding the impact of social and physical environments on PA can select from these systems that are available for free in the public domain, or design their own.Keyword(s): assessment, exercise/fitness/physical activity, research