Diaries, as people write them even today – some quite modern as blogs, many still handwritten in a notebook – have rarely sparked the interest of psychology. This is quite surprising, as they contain fascinating data that can help pose and pursue various questions. First and foremost, diaries are records of human thought, feeling, and action, written directly from the first-person perspective. They do not replace guided or narrative interviews, surveys, or observations, but they do offer another, until now underutilized methodological approach to the actual subject of psychology. In principle, nearly all topics of modern psychology – family and relationships, youth and aging, identity, work, illness, and much more – can be explored through the analysis of diaries.