Abstract: The invention relates to a modular spindle unit for forming a multi-spindle machining head (36, 47, 49) for a machine tool, wherein the spindle unit (01, 30, 54) includes at least one drive spindle (02), to which a tool (37) or tool carrier (51, 52) is attachable, and wherein the spindle unit (01, 30, 54) includes a module housing (08, 31) in which the drive spindle (02) is mounted so as to be freely rotatable and axially displaceable. The module housing (08, 31) is attachable to the machining head (36, 47, 49) together with other module housings (08, 31), wherein the spindle unit (01, 30, 54) includes at least one drive motor (03), with which a drive spindle (02) allocated thereto is drivable in rotary manner, and wherein the center axis of the drive motor (03) and the center axis of the drive spindle (02) extend coaxially and define a unit center axis (07).