Modular storage units that can be assembled without additional fasteners or tools and then coupled together with or without optional shelves into an overall storage system using a common twist-lock fastener. The modular storage units are assembled from top, bottom and side panels. The top and bottom panels for each modular storage unit are structurally identical as are the side panels. Modular storage units having different widths have commonly sized and structurally identical side panels. Storage systems that are built up from the modular storage units can be easily assembled in work vehicles and other close quarters or tight spaces. The modular storage units can include plurality of trays or other articles for storing various parts, articles, etc.