A load-handling device for lifting and moving containers stacked in stacks in a storage structure, the storage structure including, above the stacks of containers, a first set of tracks extending in a first direction and a second set of tracks extending in a second direction which is transverse to the first direction, the load-handling device being configured to move on the tracks (17, 19) above the stacks, the load-handling device comprising: a body having an upper portion and a lower portion, the upper portion being configured to house one or more operation components, the lower portion being arranged beneath the upper portion, the lower portion comprising a container-receiving space for accommodating at least part of a container; a wheel assembly arranged to support the body, the wheel assembly comprising a first set of wheels for engaging with the first set of tracks to guide movement of the load-handling device in the first direction and a second set of wheels for engaging with the second set of tracks to guide movement of the load-handling device in the second direction; a wheel-positioning mechanism for selectively engaging either the first set of wheels with the first set of tracks or the second set of wheels with the second set of tracks, the wheel-positioning mechanism being configured to raise and lower the first set of wheels and/or the second set of wheels relative to the body, thereby enabling the load-handling device to selectively move in either the first direction or the second direction across the tracks (17, 19) of the storage structure; and a container-lifting mechanism comprising a container-gripping assembly configured to releasably grip a container and a raising and lowering assembly configured to raise and lower the container-gripping assembly, characterised in that at least one wheel (35, 37) of the first and second sets of wheels (35, 37) is provided by a combined wheel and wheel-control unit comprising the at least one wheel (35, 37) and a controller configured to control rotation of the at least one wheel (35, 37), the combined wheel and wheel-control unit comprises a mounting component on which the at least one wheel (35, 37) and the controller are mounted to provide a single unit. a body having an upper portion and a lower portion, the upper portion being configured to house one or more operation components, the lower portion being arranged beneath the upper portion, the lower portion comprising a container-receiving space for accommodating at least part of a container; a wheel assembly arranged to support the body, the wheel assembly comprising a first set of wheels for engaging with the first set of tracks to guide movement of the load-handling device in the first direction and a second set of wheels for engaging with the second set of tracks to guide movement of the load-handling device in the second direction; a wheel-positioning mechanism for selectively engaging either the first set of wheels with the first set of tracks or the second set of wheels with the second set of tracks, the wheel-positioning mechanism being configured to raise and lower the first set of wheels and/or the second set of wheels relative to the body, thereby enabling the load-handling device to selectively move in either the first direction or the second direction across the tracks (17, 19) of the storage structure; and a container-lifting mechanism comprising a container-gripping assembly configured to releasably grip a container and a raising and lowering assembly configured to raise and lower the container-gripping assembly, characterised in that at least one wheel (35, 37) of the first and second sets of wheels (35, 37) is provided by a combined wheel and wheel-control unit comprising the at least one wheel (35, 37) and a controller configured to control rotation of the at least one wheel (35, 37), the combined wheel and wheel-control unit comprises a mounting component on which the at least one wheel (35, 37) and the controller are mounted to provide a single unit.