Although Pepys’s Diary is one of the earliest such works commonly read today as literature, the diary as a written and literary form goes back to antiquity, with the English term deriving from the Latin diarium, from dies, meaning day. The Meditations of the Roman emperor Marcus Aurelius has diary-like aspects, as do a number of Christian mystical works from the Middle Ages describing religious experiences and visions. However, the diary form began to flower more extensively starting in the Renaissance, with its stronger emphasis on the individual. In addition to their insight into personality and character, diaries are also valued as primary historical documents, a trait they share with letters. In some cases, a diary functions as a principal source for historians about the period in which it was written. This is true of Pepys’s work, which is frequently quoted in historical texts for its information on the Restoration, the plague, and the Great Fire of London, among other topics.

One hallmark of diaries is that they are often private and intended only for the use of the author, thus ensuring greater honesty in reporting. To better ensure privacy (as well as to allow for faster composition) Pepys’s went so far as to write his diary in a special shorthand devised in 1626 by Thomas Shelton, which Pepys also used in his business documents.