A logbook (a ship's logs or simply log) is a record of important events in the management, operation, and navigation of a ship. It is essential to traditional navigation, and must be filled in at least daily.

The term originally referred to a book for recording readings from the chip log that was used to estimate a ship's speed through the water. Today's ship's log has grown to contain many other types of information, and is a record of operational data relating to a ship or submarine, such as weather conditions, times of routine events and significant incidents, crew complement or what ports were docked at and when.

The term logbook has spread to a wide variety of other usages. Today, a virtual or electronic logbook is typically used for record-keeping for complex machines such as nuclear plants or particle accelerators. In military terms, a logbook is a series of official and legally binding documents. Each document (usually arranged by date) is marked with the time of an event or action of significance.