A lost work is a document, literary work, or piece of multimedia produced some time in the past, of which no surviving copies are known to exist. It can only be known through reference. This term most commonly applies to works from the classical world, although it is increasingly used in relation to modern works. A work may be lost to history through the destruction of an original manuscript and all later copies.

Works—or, commonly, small fragments of works—have survived by being found by archaeologists during investigations, or accidentally by anybody, such as, for example, the Nag Hammadi library scrolls. Works also survived when they were reused as bookbinding materials, quoted or included in other works, or as palimpsests, where an original document is imperfectly erased so the substrate on which it was written can be reused. The discovery, in 1822, of Cicero's De re publica was one of the first major recoveries of a lost ancient text from a palimpsest. Another famous example is the discovery of the Archimedes palimpsest, which was used to make a prayer book almost 300 years after the original work was written. A work may be recovered in a library, as a lost or mislabeled codex, or as a part of another book or codex.

Well known but not recovered works are described by compilations that did survive, such as the Naturalis Historia of Pliny the Elder or the De Architectura of Vitruvius. Sometimes authors will destroy their own works. On other occasions, authors instruct others to destroy their work after their deaths. This should have happened with several pieces, but did not, such as Virgil's Aeneid, which was saved by Augustus, and Kafka's novels, which were saved by Max Brod. Handwritten copies of manuscripts existed in limited numbers before the era of printing. The destruction of ancient libraries, whether by intent, chance or neglect, resulted in the loss of numerous works. Works to which no subsequent reference is preserved remain unknown.

Deliberate destruction of works may be termed literary crime or literary vandalism (see book burning).