Raupenschlepper Ost (German: "Caterpillar Tractor East", more commonly abbreviated to RSO) was a fully tracked, lightweight vehicle used by the Wehrmacht in World War II.

It was conceived in response to the poor performance of wheeled and half-tracked vehicles in the mud and snow during the Wehrmacht's first autumn and winter on the Soviet Front.

After the Wehrmacht's first fall and winter (1941–1942) on the Eastern Front, they found that the extremely primitive roadways in the USSR and seasonal mud required a fully tracked supply vehicle to maintain mobility.

Initially designed as a prime mover and artillery supply vehicle, it eventually served in a wide variety of roles.

It gave outstanding service due to its reliability, its ease of maintenance, and its capability to take over a variety of roles—in every kind of terrain—that other vehicles lacked.

[2] By 1943 infantry anti-tank units at the front complained strongly that it was almost impossible to move their guns using trucks at daylight under enemy fire, leading to enormous losses of equipment during "emergency relocations" (at the time a euphemism for withdrawal), and their opinions reached the top ranks.

The suspension of the RSO remained unchanged, but the front driver's compartment was replaced with a low, lightly armoured superstructure.

The German Army Group South, where the units issued for combat testing, declared the vehicle useful, and large-scale production was quickly authorised.

Despite the decision to have Steyr shift its entire production line to the RSO/PaK 40, no specific order arrived, and only the approximately 60 pre-production vehicles were ever manufactured.