The present state of the art of acousto optical spectrometer (AOS) development for spectroscopy is reviewed with special emphasis given to the University of Cologne built systems. The underlying principles of acousto optical detection are discussed and are used for an in depth analysis of limitations and an optimal use of this powerful spectroscopic tool. The inherent instabilities were traced to arise from laser “speckles” which have been the primary cause of AOS instabilities, producing severe baseline problems and thus limiting its performance. The role of laser scatter is discussed and suitable techniques for removal of scattered background laser light are described. In addition, the influence of the various optical components on the performance of AOS is evaluated. Two examples of Cologne built AOS are described in detail. One is a low resolution AOS with 1.1 GHz bandwidth now in operation at the ESO SEST telescope at La Silla, Chile, and the other is a medium resolution AOS with 287 MHz bandwidth stationed at the KOSMA 3-m-radiotelescope at Gornergrat near Zermatt, Switzerland. Technical details of the high resolution AOS at Gornergrat are also given. The AOS are shown to be extremely stable so that they can be used for more than 100 sec integration time on each position in the position switch mode as proved by the spectroscopic Allan variance, specifically developed as a critical performance test procedure. Experimental results are presented and a future outlook for further AOS developments is given, i.e. array AOS or variable bandwidth AOS.