Method of positioning a mobile X-ray detector unit of an X-ray system, X-ray system and mobile X-ray detector unit.method is described for positioning a mobile X-ray detector unit (25, 26) of an X-ray system (1). The X-ray system (1) deployed has X-ray radiation units (5, 6, 7) and at least one mobile X-ray detector unit (25, 26). The X-ray radiation units (5, 6, 7) thereby each have an active and a passive operating status. The X-ray radiation units (5, 6, 7) are each assigned a detector holder (8, 9, 10, 11, 12) to hold a mobile X-ray detector unit (25, 26). To generate X-ray recordings, X-ray radiation emitted by an active X-ray radiation unit (5, 6, 7) is detected by means of a mobile X-ray detector unit (25, 26), which is located in a detector holder (8, 9, 10, 11, 12) of said X-ray radiation unit (5, 6, 7). A mobile X-ray detector unit (25, 26) is positioned as a function of the operating status of the X-ray radiation units (5, 6, 7) such that a mobile X-ray detector unit (25, 26) is located in each of the detector holders (8, 9, 10, 11, 12) assigned to the active X-ray radiation units (5, 6, 7). A location unit is used to determine the location of a mobile X-ray detector unit automatically.