This chapter presents an overview of the four basic direct observation (DO) data collection tactics, their key advantages and limitations, protocols used for observer training, and possible sources of observer error. The four main tactics include: Event Recording (ER), Duration Recording (DR), Interval Recording (IR), and Momentary Time Sampling (MTS). A major advantage of DO is that it is a direct method of data collection that produces objective information with strong internal (or face) validity. Research in which DO instruments are used provides evidence not only about PA but also about concurrent contextual variables. The contextual categories within the systems are setting-specific (e.g. physical education classes, school campuses, school recess periods, parks) and users will need to select the DO instrument that best fits their interest and the setting where data are to be collected. The contextual categories within the instruments all have strong face validity, and sometimes they can be used with other DO systems.