Monopoly was first commercially released by Parker Brothers in 1935.
Parker Brothers acquired the rights to Charles Darrow's version and began selling the game during the Great Depression. The release turned Monopoly into a major commercial success and established the familiar board, property names, money, cards, and playing pieces.
The year 1935 refers to Parker Brothers' commercial release of the modern Monopoly game, not to the invention of every idea behind it. The game's design developed from earlier games, especially Elizabeth Magie's The Landlord's Game, which was patented in 1904.
Another frequent confusion concerns Darrow's role. He is credited with popularizing and commercially developing the version sold by Parker Brothers, while the game's broader ancestry includes earlier designers and players. Thus, 1935 marks the key mass-market launch of modern Monopoly.