Size, position and stability of the synchrotron radiation source BESSY are determined at present by imaging optical systems and wire monitors in the beamlines and by electrostatic position monitors in the storage ring. Since the size of the source has been brought down to FWHM-values Δy = 0.15 mm in vertical and Δx = 0.4 mm in horizontal directions for stored currents of IR = 200 mA, the requirements with respect to the stability of position and size have increased considerably. Vertical position changes of 0.1 mm lead to apparent energy shifts of up to 100 meV on monochromators without entrance slits. Even when entrance slits are used, the intensity and degree of polarization of the radiation depend on the stability of the position of the source. All monitors in the beam lines and the monochromators themselves are sensitive to thermal expansion of their optical components. This means, that slow shifts occur during the life time of one filling of the storage ring and sudden shifts during each injection (between closing and opening of the beam shutters). Apart from these long time effects, flaring of the image of the source in the frequency range of a few Hz becomes visible on the optical system from time to time. In order to separate ring problems from beam line problems, such monitor systems are indispensable. Improvement of the monitors and measurements with a random access camera will be presented and discussed.