In psychology and philosophy stream of consciousness, introduced by William James, is the set of constantly changing inner thoughts and sensations which an individual has while conscious, used as a synonym for stream of thought.

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In literary criticism, stream of consciousness denotes a literary technique which seeks to describe an individual's point of view by giving the written equivalent of the character's thought processes. Stream-of-consciousness writing is strongly associated with the modernist movement. Its introduction in the literary context, transferred from psychology, is attributed to May Sinclair.

With its rapid, unconnected association of objects, geometrical shapes and numerology Sir Thomas Browne's Discourse The Garden of Cyrus (1658) may, upon examination of its text, be considered one of the very earliest examples of stream-of-consciousness writing. Another would be The Life and Opinions of Tristram Shandy, Gentleman (or, more briefly, Tristram Shandy) by Laurence Sterne, (1760). Both of these are preceded by Ovid's Metamorphoses in ancient Rome. Further examples of the development of this style are The Narrative of Arthur Gordon Pym of Nantucket by Edgar Allen Poe (1837/1838) and Édouard Dujardin 's Les Lauriers sont coupes (1888). Tolstoy used something similar to the stream-of-consciousness technique in Anna Karenina (1877) in the portions leading to Anna's suicide; another early example is Arthur Schnitzler's 1900 short story Leutnant Gust.

A few of the most famous works to employ the technique are James Joyce's Ulysses (in particular Molly Bloom's soliloquy), Virginia Woolf's Mrs. Dalloway, William Faulkner's The Sound and the Fury and Jack Kerouac's On the Road.

Brian W. Aldiss' 1969 novel Barefoot in the Head employs a stream of consciousness style as a necessary part of the plot. The leading character, a Serbian named Charteris, wanders through a Europe aerosol-bombed with a persistent psychedelic chemical agent. A war between Europe and an "Arab coalition" has resulted in the destruction of the framework of European society by the effects of the hallucinogenic chemical weapon. Europeans are consequently on a permanent acid trip and are only able to think in streams of lateral associations of tangential ideas.

Stream of consciousness writing represents a bridge between prose and poetry.

The technique has also been parodied, notably by David Lodge in the final chapter of The British Museum Is Falling Down. Stream-of-consciousness writing is characterised by associative leaps that can make the prose difficult to follow. Typically, writers employ very long sentences which move from one thought to another. Sometimes, writers avoid punctuation altogether in order to prevent artificial breaks in the "stream."