A cantilever shelving system comprising: a frame structure comprising a plurality of vertical support members, a lateral support structure interconnecting the vertical support members, and a base; a cantilever shelf support structure mounted to the frame structure; and a plurality of generally rectangular, load-bearing shelves supported by the cantilever shelf support structure and the frame structure, wherein each of the shelves comprises an upper support surface and a plurality of mounting collars, wherein a mounting collar is located near each corner of the shelf; wherein the cantilever shelf support structure comprises: a plurality of horizontally extending cantilever support arms, at least one cantilever support arm mounted to each of the vertical support members; and a plurality of shelf support fittings, at least two shelf support fittings mounted to each of the cantilever support arms; wherein each cantilever support arm comprises: an elongated portion extending from a proximal end to a distal end along a longitudinal axis and having a planar upper wall and opposing side walls; and a mounting flange located at the proximal end and including opposing side braces spaced laterally apart to enable the mounting flange to closely fit on a vertical support member; and at least two slots included in each of the side walls, wherein at least one slot is near the proximal end and at least one slot is near the distal end; wherein each shelf support fitting comprises: a mounting portion comprising at least one mounting tab and at least one lock notch; and a support portion comprising an upward projection laterally offset from the mounting portion; wherein the at least one mounting tab is inserted into one of the slots and engages an inside surface of a side wall of the cantilever support arm above the one slot and the lock notch engages a lower end of the one slot; and wherein the mounting collars of the load-bearing shelves engage the upward projections of the shelf support fittings; wherein the shelves are positioned laterally adjacent to cantilever support arms and the shelf support surfaces are coplanar with the upper walls of the cantilever support arms. a frame structure comprising a plurality of vertical support members, a lateral support structure interconnecting the vertical support members, and a base; a cantilever shelf support structure mounted to the frame structure; and a plurality of generally rectangular, load-bearing shelves supported by the cantilever shelf support structure and the frame structure, wherein each of the shelves comprises an upper support surface and a plurality of mounting collars, wherein a mounting collar is located near each corner of the shelf; wherein the cantilever shelf support structure comprises: a plurality of horizontally extending cantilever support arms, at least one cantilever support arm mounted to each of the vertical support members; and a plurality of shelf support fittings, at least two shelf support fittings mounted to a plurality of horizontally extending cantilever support arms, at least one cantilever support arm mounted to each of the vertical support members; and a plurality of shelf support fittings, at least two shelf support fittings mounted to each of the cantilever support arms; wherein each cantilever support arm comprises: an elongated portion extending from a proximal end to a distal end along a longitudinal axis and having a planar upper wall and opposing side walls; and a mounting flange located at the proximal end and including opposing side braces spaced laterally apart to enable the mounting flange to closely fit on a vertical support member; and at least two slots included in each of the side walls, wherein at least one slot is near the proximal end and at least one slot is near the distal end; wherein each shelf support fitting comprises: a mounting portion comprising at least one mounting tab and at least one lock notch; and a support portion comprising an upward projection laterally offset from the mounting portion; wherein the at least one mounting tab is inserted into one of the slots and engages an inside surface of a side wall of the cantilever support arm above the one slot and the lock notch engages a lower end of the one slot; and wherein the mounting collars of the load-bearing shelves engage the upward projections of the shelf support fittings; an elongated portion extending from a proximal end to a distal end along a longitudinal axis and having a planar upper wall and opposing side walls; and a mounting flange located at the proximal end and including opposing side braces spaced laterally apart to enable the mounting flange to closely fit on a vertical support member; and at least two slots included in each of the side walls, wherein at least one slot is near the proximal end and at least one slot is near the distal end; a mounting portion comprising at least one mounting tab and at least one lock notch; and a support portion comprising an upward projection laterally offset from the mounting portion; wherein the at least one mounting tab is inserted into one of the slots and engages an inside surface of a side wall of the cantilever support arm above the one slot and the lock notch engages a lower end of the one slot; and wherein the mounting collars of the load-bearing shelves engage the upward projections of the shelf support fittings; wherein the shelves are positioned laterally adjacent to cantilever support arms and the shelf support surfaces are coplanar with the upper walls of the cantilever support arms.