Mechanoids are the colonizing robots used by Earth to find and prepare planets for human occupation. Constructed during the first few years of Earth's expansionist period, the Mechanoids have largely been forgotten, and the commands needed to gain access to the beautiful cities they create lost by man. When anyone encounters a Mechanoid city, the robots will confine him indefinitely until they are given the codes that they should receive from the colonists. Such prisoners will be well catered for, but it is a sterile existence.

On the planet Mechanus, the Daleks attacked and largely destroyed a Mechanoid city in their attempts to catch and exterminate the Doctor. Mechanoids, however, are used to repairing each other, and have infinite patience to rebuild their work should it be destroyed.

A Mechanoid is about 1.7 metres tall and looks like a multi-faceted, angular sphere on a low circular base. The metal triangular panels that comprise its outer skin conceal circuits, sensors and its weaponry. At the top of the sphere, a small column topped by a broad, flat metal cone sticks out: the column contains the Mechanoid's main audio sensors. A narrow metal band runs round the middle of the robot and houses the two, slender metal feelers that the Mechanoid uses to operate machinery. Mechanoids have grating, electronic voices, and precede and finish their simple commands with spoken binary sequences.