A picking device for use on a grid-based storage system, the picking device being adapted so as to operate on the grid-based storage system, the storage system comprising: a first set of parallel rails or tracks and a second set of parallel rails or tracks extending substantially perpendicularly to the first set of tracks or rails in a substantially horizontal plane to form a grid pattern comprising a plurality of grid spaces; a plurality of containers located beneath the tracks or rails within a footprint of a single grid space, the picking device being adapted to move laterally on the rails or tracks above the containers, at least one container 10comprising inventory items; wherein the picking device comprises picking means adapted to pick at least one inventory item from at least one container, the picking device further comprising means for moving the or each item picked by the picking means from the at least one container to a different predetermined container (10, DT), the picking device comprising at least one vehicle means, each vehicle means comprising a vehicle body and a wheel assembly, the 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 a 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 a second direction such that the picking device may be positioned above a predetermined container in order to pick the required inventory item, wherein the picking device comprises load handling means for lifting and moving at least one container, the picking device further comprising a container-receiving space and lifting means (38, 39) for lifting at least one container from beneath the grid in to the container receiving space, in which the picking means, the load handling means and the lifting means (38, 39) are co-located within a vehicle body. a first set of parallel rails or tracks and a second set of parallel rails or tracks extending substantially perpendicularly to the first set of tracks or rails in a substantially horizontal plane to form a grid pattern comprising a plurality of grid spaces; a plurality of containers located beneath the tracks or rails within a footprint of a single grid space, the picking device being adapted to move laterally on the rails or tracks above the containers, at least one container 10comprising inventory items; wherein the picking device comprises picking means adapted to pick at least one inventory item from at least one container, the picking device further comprising means for moving the or each item picked by the picking means from the at least one container to a different predetermined container (10, DT), the picking device comprising at least one vehicle means, each vehicle means comprising a vehicle body and a wheel assembly, the 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 a 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 a second direction such that the picking device may be positioned above a predetermined container in order to pick the required inventory item, wherein the picking device comprises load handling means for lifting and moving at least one container, the picking device further comprising a container-receiving space and lifting means (38, 39) for lifting at least one container from beneath the grid in to the container receiving space, in which the picking means, the load handling means and the lifting means (38, 39) are co-located within a vehicle body.