A self-propelled modular transporter (SPMT) is a platform vehicle with a large array of wheels. SPMTs are used for transporting massive objects such as large bridge sections, oil refining equipment, motors and other objects that are too big or heavy for trucks. Trucks can however provide traction and braking for the SPMTs on inclines and descents.

A typical SPMT has a grid of computer-controlled axles, usually 2 axles across and 4-8 axles along. When two (or more) axles are placed side-by-side, this is called an axle line. All axles are individually controllable, in order to evenly distribute weight and steer accurately. Each axle can swivel through 360°. The axles are coordinated by the control system to allow the SPMT to turn, move sideways or even spin in place.