Abstract: A security device for personal computers, computer peripherals and similar valuable but readily portable units of equipment includes a tray secured to a working surface by bolts on which the unit is to be used, the tray being of a size to accommodate the unit and having front and back flanges and upward extension from its side walls to restrain the unit from horizontal movement without interference with its functions, the extensions being linked by a normally locked releasable cross bar placed so as to retain the unit in the tray covering the bolts, again without frictional interference with the unit. The tray of one device may be bolted to the cross bar of another security device, as when a monitor is stacked on a computer. The cross bar is preferably hinged to one upward extension and secured to the other by a tamper resistant screw, access to which is through a housing blocked by a removable lock plug and having an axis which is not coincident with that of the screw.