A waldo is an electronic telemetric input device Muppeteers use to remotely perform characters. Shaped like a Muppet's head, the waldo sends radio signals to a mechanism in the puppet that moves the puppet's mouth in sync with the waldo's mouth. Waldos are sometimes used in conjunction with a computer interface that provides enhanced control of the puppet.
The term "waldo" comes from a 1942 novella written by science-fiction author Robert A. Heinlein. In the novella, a disabled scientist named Waldo builds a robot to amplify his limited abilities. As a tribute to Heinlein, NASA engineers adopted the name "waldo" for many of their early robot control devices, a tradition which was carried on by the Muppets when they began using similar devices.
Waldoes were used prominently on Fraggle Rock (for the Doozers and the Gorgs' heads). They were also used in most Muppet movies, The Jim Henson Hour, and various Creature Shop productions. The performer at the waldo usually provides the voice for the character.
Waldo C. Graphic, the computer generated Muppet, is performed entirely with a waldo; the device also inspired his name.
Waldo was a generic, commonly used term for puppet and animatronic control devices that was used by a variety of different companies and organizations including the Muppets until it was appropriated and registered as a trademark by the Character Shop, a California-based creature effects company. Because of this, while the use of waldo-like devices is widespread in puppetry and visual effects, the Character Shop is the only company in the United States permitted to use the name "Waldo" commercially for puppet control devices.