"The Interpretation of Dreams" is a book by the psychoanalyst Sigmund Freud first published in 1990. Today it is considered one of the most significant written works of XX century and it is among the most influential books ever published.
The book introduces Freud's theory of the unconscious with respect to dream interpretation. Dreams, in Freud's view, were all forms of "wish fulfillment" — attempts by the unconscious to resolve a conflict of some sort, whether something recent or something from the recesses of the past. However, because the information in the unconscious is in an unruly and often disturbing form, a "censor" in the preconscious will not allow it to pass unaltered into the conscious. During dreams, the preconscious is more lax in this duty than in waking hours, but is still attentive: as such, the unconscious must distort and warp the meaning of its information to make it through the censorship. As such, images in dreams are often not what they appear to be, according to Freud, and need deeper interpretation if they are to inform on the structures of the unconscious.