"The Diary" is the thirtieth episode of the sixth season of the American animated television series Adventure Time.

It was written by Canadian illustrator Jillian Tamaki, her second episode on the show after "Astral Plane".

[1] Her graphic novel This One Summer won the Caldecott Medal—the first of its format to receive this award—earlier in the month of the episode's broadcast.

[3] Regarding how she first got on the show, Tamaki said that the Adventure Time crew knew of her comics and invited her to write for an episode with them.

[4] Regarding her method of writing, Eric Thurm of Wired wrote that the missing pages of the diary in the episode allowed for Tamaki to "effectively build an Adventure Time quest out of an afterschool drama".

In her comics career, the amount of drawings necessary for the field helped her as a storyboard artist on the episode, Tamaki explained.

He praised Mintz's voice acting as bringing "immediate credibility to the story", while the inclusion of Jake contributed to the season's theme of parenting.

Club graded the episode with an A, likewise complimenting Mintz's role while describing Shawkat's portrayal of Betsy as "a moving portrait of a shy, bookish young girl that falls desperately in love and loses herself in the process".