Diary methods refer to a broad set of methodological tools for conducting research based on in situ records of events, states, and/or reflections. Diary methods are a form of naturalistic observation and rely heavily on the participant as observer, whether a parent who observes and records an infant’s first words, a child who responds to a prompt asking him to rate his emotional state at multiple points during the school day, or an older adult who records activities and sleep patterns throughout the week. Diary methods often capture aspects of daily life that cannot be studied in the laboratory, including spontaneous behavior, private behavior, contextual factors, and rare or unpredictable events. For this reason, diary methods are especially useful for identifying the onset of particular ...