The doll diaries are one of the two main pieces of extras included with the Monster Highdolls, the other being the profiles, and one of the three main pieces of fictional support of the toys and merchandise. The diaries together form their own continuity, though one that lightly overlaps with the cartoon continuity.
Not all dolls come with diaries. Those of the larger main line are guaranteed to come with diaries, those available at the San Diego Comic-Con International have been packaged with diaries so far, and all party-themed doll lines have mini-diaries included.
