A clamping apparatus for attaching work pieces to be machined to the work table of a machine tool in a precisely defined position comprises at least one clamping rail to be attached to the work table of the machine, as well as at least one work piece holder. The clamping rail is provided with an elongate protruding portion, the front face thereof constituting a stop surface and which defines a recess between itself and the work table of the machine tool. The elongate protruding portion may be provided with threaded bores for receiving clamping screws. Each work piece holder includes a clamping portion which can be inserted into the recess and clamped therein by means of the clamping screws. The clamping rail includes a flat bottom surface resting on the surface of the work table of the machine tool and running perpendicular to the aforementioned front face of the elongate protruding portion. The side of the work piece holder having the aforementioned clamping portion includes a reference surface adapted to rest against the front face of the elongate portion of the clamping rail.