The diary method is a longitudinal qualitative research approach in which participants document their experiences, behaviors, and attitudes as they unfold in natural settings, rather than recalling them retrospectively in a single session. Within research operations, diary studies are valued for capturing in-the-moment context: how a product fits into a daily routine, how opinions shift across a purchase journey, or how usage patterns evolve over weeks. Because participants self-report at the point of experience, the diary method reduces recall bias and surfaces the kind of lived detail that traditional depth interviews often miss. It is particularly well suited to category immersion, in-home usage testing, and longitudinal brand tracking.