A robotic storage and retrieval system comprising: a shelving unit including; at least one vertical column of a plurality of vertically arranged storage locations, each storage location sized to receive a container; a pair of vertical rails (110, 112) associated with each of the at least one vertical column; and a robotic retriever including; a drive mechanism including drive wheels for propelling the retriever to a position adjacent a lowermost one of the storage locations of the shelving unit; a recognition unit configured to recognize the unique identifier of each of the plurality of storage locations; a traction drive mechanism adapted to engage the pair of vertical rails (110, 112) to propel the retriever vertically along the at least one vertical column; a retriever apparatus operable to engage a container to move the container between a storage location and the cavity of the robotic retriever; and a processor operable to execute instructions to control the drive mechanism and traction drive mechanism to drive the retriever to a storage location according to the unique identifier as recognized by the recognition unit, and to operate the retriever apparatus to move a container between the storage location and the cavity of the robotic retriever, characterised in that the shelving unit includes a unique identifier associated with each of the plurality of storage locations; the robotic retriever defines a cavity sized to receive the container; and the retriever includes traction wheels adjacent to the drive wheels that can be driven by the same drive mechanism as the drive wheels and in that the traction wheels and drive wheels share a common axle. a shelving unit including; at least one vertical column of a plurality of vertically arranged storage locations, each storage location sized to receive a container; a pair of vertical rails (110, 112) associated with each of the at least one vertical column; and a robotic retriever including; a drive mechanism including drive wheels for propelling the retriever to a position adjacent a lowermost one of the storage locations of the shelving unit; a recognition unit configured to recognize the unique identifier of each of the plurality of storage locations; a traction drive mechanism adapted to engage the pair of vertical rails (110, 112) to propel the retriever vertically along the at least one vertical column; a retriever apparatus operable to engage a container to move the container between a storage location and the cavity of the robotic retriever; and a processor operable to execute instructions to control the drive mechanism and traction drive mechanism to drive the retriever to a storage location according to the unique identifier as recognized by the recognition unit, and to operate the retriever apparatus to move a container between the storage location and the cavity of the robotic retriever, characterised in that the shelving unit includes a unique identifier associated with each of the plurality of storage locations; the robotic retriever defines a cavity sized to receive the container; and the retriever includes traction wheels adjacent to the drive wheels that can be driven by the same drive mechanism as the drive wheels and in that the traction wheels and drive wheels share a common axle. a drive mechanism including drive wheels for propelling the retriever to a position adjacent a lowermost one of the storage locations of the shelving unit; a recognition unit configured to recognize the unique identifier of each of the plurality of storage locations; a traction drive mechanism adapted to engage the pair of vertical rails (110, 112) to propel the retriever vertically along the at least one vertical column; a retriever apparatus operable to engage a container to move the container between a storage location and the cavity of the robotic retriever; and a processor operable to execute instructions to control the drive mechanism and traction drive mechanism to drive the retriever to a storage location according to the unique identifier as recognized by the recognition unit, and to operate the retriever apparatus to move a container between the storage location and the cavity of the robotic retriever, characterised in that the shelving unit includes a unique identifier associated with each of the plurality of storage locations; the robotic retriever defines a cavity sized to receive the container; and the retriever includes traction wheels adjacent to the drive wheels that can be driven by the same drive mechanism as the drive wheels and in that the traction wheels and drive wheels share a common axle.