"In the Pines", aka "Where Did You Sleep Last Night?" & "My Girl", is a traditional American folk song originating from two songs, "In the Pines" and "The Longest Train", both of whose authorship is unknown and date back to at least the 1870s. The songs originated in the Southern Appalachian area of the United States in the contiguous areas of Eastern Tennessee and Kentucky, Western North Carolina and Northern Georgia.

Versions of the song have been recorded by many artists in numerous genres, but it is most often associated with American bluegrass musician Bill Monroe and American Blues musician Lead Belly, both of whom recorded very different versions of the song in the 1940s and 1950s.

A version of the song performed by The Four Pennies reached the UK top-twenty in 1964. A live performance by the American grunge band Nirvana reinterpreted Lead Belly's version and was recorded during their MTV Unplugged performance in 1993.

Like numerous other folk songs, "In the Pines" was passed on from one generation and locale to the next by word of mouth. In 1925, a version of the song was recorded onto phonograph cylinder by a folk collector. This was the first documentation of "The Longest Train" variant of the song, which includes a verse about "The longest train I ever saw". This verse probably began as a separate song that later merged into "In the Pines". Lyrics in some versions about "Joe Brown's coal mine" and "the Georgia line" may refer to Joseph E. Brown, a former Governor of Georgia, who famously leased convicts to operate coal mines in the 1870s. While early renditions which mention the head in the "driver's wheel" make clear that the decapitation was caused by the train, some later versions would omit the reference to the train and reattribute the cause. As music historian Norm Cohen pointed out in his 1981 book, Long Steel Rail: The Railroad in American Folksong, the song came to consist of three frequent elements: a chorus about "in the pines", a verse about "the longest train" and a verse about a decapitation, but not all elements are present in all versions.

Starting in 1926, commercial recordings of the song were made by various folk and bluegrass bands. In her 1970 Ph.D. dissertation, Judith McCulloh found 160 permutations of the song. As well as rearrangement of the three frequent elements, the person who goes into the pines, or who is decapitated, is described as a man, woman, adolescent, husband, wife, or parent, while the pines can be seen as representing sexuality, death, or loneliness. The train is described as killing a loved one, as taking one's beloved away, or as leaving an itinerant worker far from home.

In variants in which the song describes a confrontation, the person being challenged is always a woman. The folk version by the Kossoy Sisters asks, "Little girl, little girl, where'd you stay last night? Not even your mother knows." The reply to the question, "Where did you get that dress/ And those shoes that are so fine?" from one version is, "From a man in the mines/Who sleeps in the pines." The theme of a woman being caught doing something she should not is thus also common to many variants. One variant, performed in the early twentieth century by the Ellison clan (Ora Ellison, deceased) in Lookout Mountain, Georgia, tells of a young Georgia girl who flees to the pines after being raped. Her rapist, a male soldier, is later beheaded by the train.

Some versions of the song also reference the Great Depression, with the "black girl" being a hobo on the move from the police, who witnesses the murder of her father while train-jumping. She hides from this by sleeping in the pines, in the cold.