Abstract: The disclosure relates to aisle width sensor for a mobile storage system for determining the width between adjacent movable storage units, particularly when creating an aisle therebetween. The sensor includes a first bracket which is slidable with respect to a second bracket. The first and second brackets are attached to respective first and second arms of a scissor arm arrangement between the adjacent storage units. The sensor provides an output, particularly an analog output, which is proportional to the distance between the adjacent storage units and therefore simple to calculate and process for the central controller of the mobile storage system.