A load-handling device for lifting and moving containers stacked in stacks in a storage system, the system comprising a plurality of rails or tracks arranged in grid pattern above the stacks of containers, the grid pattern comprising a plurality of grid spaces, each stack being located within a footprint of only a single grid space, the load handling device being configured to move laterally on the rails or tracks above the stacks, the load handling device further comprising a lifting mechanism, the lifting mechanism comprising a gripper device, the gripper device being configured to grip a container from above, wherein the load-handling device comprises: a vehicle comprising a body having an upper portion and a lower portion: the upper portion for housing components such as power components, and/or control components, and/or drive components and/or lifting components; the lower portion arranged directly beneath the upper portion, the lower portion comprising a cavity for accommodating a container and a wheel assembly, the wheel assembly comprising a first set of wheels for engaging with a first set of rails or tracks to guide movement of the device in a first direction and a second set of wheels for engaging with a second set of rails or tracks to guide movement of the device in a second direction, wherein the second direction is transverse to the first direction; and the lifting mechanism being configured to raise and lower the gripper device relative to the cavity, the lifting mechanism being located above the cavity, the wheel assembly further comprises a wheel positioning mechanism wherein, the wheel positioning mechanism comprising means for selectively engaging either the first set of wheels with the first set of rails or tracks or the second set of wheels with the second set of rails or tracks, characterised in that: the means for selectively engaging comprises four separate moving means, each of the four separate moving means being on each side of the load handling device configured to act on a respective half a set of wheels (116, 118), thereby enabling the load handling device to selectively move in either the first or second direction across the grid, wherein the wheel positioning mechanism further comprises an arm pivotally mounted at its outer end to each of the wheels, the mechanism further comprising two linkages, the inner end of each arm being connected to a linkage, the upper ends of both linkages being connected to the lower end of a common linkage, the four separate moving means comprising motor means, each configured to act on the common linkage, the motor means configured to act so as to draw the common linkage upwardly. the upper portion for housing components such as power components, and/or control components, and/or drive components and/or lifting components; the lower portion arranged directly beneath the upper portion, the lower portion comprising a cavity for accommodating a container and a wheel assembly, the wheel assembly comprising a first set of wheels for engaging with a first set of rails or tracks to guide movement of the device in a first direction and a second set of wheels for engaging with a second set of rails or tracks to guide movement of the device in a second direction, wherein the second direction is transverse to the first direction; and the lifting mechanism being configured to raise and lower the gripper device relative to the cavity, the lifting mechanism being located above the cavity, the wheel assembly further comprises a wheel positioning mechanism wherein, the wheel positioning mechanism comprising means for selectively engaging either the first set of wheels with the first set of rails or tracks or the second set of wheels with the second set of rails or tracks,