The Interpretation of Dreams is a book by Sigmund Freud. The first edition was first published in German in November 1899 as Die Traumdeutung (though post-dated as 1900 by the publisher). The publication inaugurated the theory of Freudian dream analysis, which activity Freud famously described as "the royal road to the understanding of unconscious mental processes".
Finally, Freud established a direct connection between dreams and insanity, between the symbolic visions of our sleep and the symbolic actions of the mentally deranged.
Fourthly, Freud showed that sexual desires play an enormous part in our unconscious, a part which puritanical hypocrisy has always tried to minimize, if not to ignore entirely.