Fluorescent dyes are required for many functional and structural investigations in the life sciences. For example, they are attached as markers to structures to be observed and can thus visualize inter- and intramolecular interactions and dynamics by means of imaging methods. Depending on their radiation behavior, they can thus form the basis for ultra-high-resolution microscopy of biological and synthetic structures. On th other hand, their photo physics determines the quality of classical fluorescence microscopy investigations. The elucidation and understanding of the properties underlying the fluorescence of the dyes under various conditions (such as temperature, pressure, and the chemical environment) is therefore of central importance.Our dedicated custom built fluorescence microscopes and analysis methods enable us to examine the emission properties of individual dye molecules and provide access to a range of photophysical parameters.