A storage system comprising: a first set of parallel rails or tracks and a second set of parallel rails or tracks extending transverse to the first set in a substantially horizontal plane to form a grid pattern comprising a plurality of grid spaces; a plurality of stacks of containers located beneath the rails, and arranged such that each stack occupies a footprint of a single grid space; a load handling device arranged to move laterally above the stacks on the rails, the load handling device comprising a container-receiving space located above the rails and a lifting device arranged to lift a single container from a stack into the container-receiving space; wherein the load handling device is configured to move in a first direction along a first set of rails or tracks and in a second direction along a second set of rails or tracks, wherein the second direction is transverse to the first direction; wherein the load handling device comprises a wheel assembly comprising a first set of wheels for engaging with the first set of rails or tracks to guide movement of the device in the first direction and a second set of wheels for engaging with the second set of rails or tracks to guide movement of the device in the second direction; characterised in that the load handling device comprises an external housing that substantially encloses the container-receiving space, the wheels are arranged around a periphery of the container-receiving space, and the external housing extends no further than the wheels such that the load handling device has a footprint that occupies only a single grid space in the storage system. a first set of parallel rails or tracks and a second set of parallel rails or tracks extending transverse to the first set in a substantially horizontal plane to form a grid pattern comprising a plurality of grid spaces; a plurality of stacks of containers located beneath the rails, and arranged such that each stack occupies a footprint of a single grid space; a load handling device arranged to move laterally above the stacks on the rails, the load handling device comprising a container-receiving space located above the rails and a lifting device arranged to lift a single container from a stack into the container-receiving space; wherein the load handling device is configured to move in a first direction along a first set of rails or tracks and in a second direction along a second set of rails or tracks, wherein the second direction is transverse to the first direction; wherein the load handling device comprises a wheel assembly comprising a first set of wheels for engaging with the first set of rails or tracks to guide movement of the device in the first direction and a second set of wheels for engaging with the second set of rails or tracks to guide movement of the device in the second direction; characterised in that the load handling device comprises an external housing that substantially encloses the container-receiving space, the wheels are arranged around a periphery of the container-receiving space, and the external housing extends no further than the wheels such that the load handling device has a footprint that occupies only a single grid space in the storage system.