A beat 'em up is a video game genre featuring hand-to-hand combat against a large number of opponents. Traditional beat 'em ups take place in scrolling, two-dimensional (2D) levels, while a number of modern games feature more open three-dimensional (3D) environments with yet larger numbers of enemies. The gameplay tends to follow arcade genre conventions, such as being simple to learn but difficult to master, and the combat system tends to be more highly developed than other side-scrolling action games. Two-player cooperative gameplay and multiple player characters are also hallmarks of the genre. Most of these games take place in urban settings and feature crime-fighting and revenge-based plots, though some games may employ historical, science fiction or fantasy themes.