A storage system comprising: a first set of parallel rails or tracks extending in an X-direction, and a second set of parallel rails or tracks extending in a Y-direction 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 is located within a footprint of a single grid space; and a multiplicity of load handling devices, wherein each 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 X-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 Y-direction, such that each load handling device is configured to selectively move laterally in the X and Y directions, above the stacks on the rails, characterised in that each load handling device comprises a container-receiving space located above the rails or tracks, for accommodating a container from the stacks, and each load handling device comprises a lifting device arranged to lift a container from a stack into the container-receiving space, each load handling device comprises an external housing that is shaped substantially in a cuboid having two sides facing the X direction, two sides facing the Y-direction and a top facing the Z-direction, such that the external housing substantially encloses the container-receiving space from above and on all four sides of the load handling device, a side of the external housing facing the Y direction extends no further, in the Y direction, than the first set of wheels on that side of the load handling device, and a side of the external housing facing the X direction extends no further, in the X direction, than the second set of wheels on that side of the load handling device, such that a load handling device occupying a grid space does not obstruct a load handling device occupying or traversing an adjacent grid space in the X direction and does not obstruct a load handling device occupying or traversing an adjacent grid space in the Y direction. a first set of parallel rails or tracks extending in an X-direction, and a second set of parallel rails or tracks extending in a Y-direction 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 is located within a footprint of a single grid space; and a multiplicity of load handling devices, wherein each 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 X-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 Y-direction, such that each load handling device is configured to selectively move laterally in the X and Y directions, above the stacks on the rails, characterised in that each load handling device comprises a container-receiving space located above the rails or tracks, for accommodating a container from the stacks, and each load handling device comprises a lifting device arranged to lift a container from a stack into the container-receiving space, each load handling device comprises an external housing that is shaped substantially in a cuboid having two sides facing the X direction, two sides facing the Y-direction and a top facing the Z-direction, such that the external housing substantially encloses the container-receiving space from above and on all four sides of the load handling device, a side of the external housing facing the Y direction extends no further, in the Y direction, than the first set of wheels on that side of the load handling device, and a side of the external housing facing the X direction extends no further, in the X direction, than the second set of wheels on that side of the load handling device, such that a load handling device occupying a grid space does not obstruct a load handling device occupying or traversing an adjacent grid space in the X direction and does not obstruct a load handling device occupying or traversing an adjacent grid space in the Y direction.