Direct behavioral observation is used to collect data describing behaviors that unfold over time. This powerful research approach is used to study health experiences across the lifespan, especially among individuals who cannot speak for themselves, including infants and young children, persons with mental impairments and disabilities, elderly with dementia, and critically ill patients. Basic principles and strategies can be extended to observation of social interactions among dyads, such as patients and caregivers, or among members of larger groups, such as families. Direct observation is used in laboratory and naturalistic settings such as homes, hospital units, and schools. Direct observation in finely grained time provides detailed information about change in a behavior and behavioral sequences over time. In intervention studies, serial measurements obtained via direct observation provides information about mechanisms effecting change.