The Rasnik alignment system was developed initially in 1983 for the monitoring of the alignment of the muon chambers of the L₃ Muon Spectrometer at CERN. Since then, the development has continued as new opto-electronic components become available. Rasnik systems are 3-point optical displacement monitors and their precision ranges from below nanometers to several micrometers, depending on the design and requirements of the systems. A result, expressed in the range/precision ratio of 2 × 10⁶, is presented. According to the calculations of the Cram'er-Rao limit, and by means of MonteCarlo simulations, a typical Rasnik image should have enough information to reach deep sub-nanometer precision. This paper is an overview of the technological developments and achievements since Rasnik was applied in high energy physics experiments.