1) The genre of zombie horror began in the 1920s with films like The Cabinet of Dr. Caligari featuring sleepwalking characters that displayed zombie-like traits.
2) In the 1930s and 1940s, films like White Zombie and I Walked with a Zombie introduced the concept of voodoo zombies and rituals.
3) George A. Romero's 1968 film Night of the Living Dead defined many of the characteristics that became standard in modern zombie films like slow, shambling zombies and a scientific cause for the outbreak.