Fluorescence correlation spectroscopy (FCS) was developed in order to char- acterize the dynamics of molecular processes in systems in thermodynamic equilibrium. FCS determines transport and chemical reaction rates from mea- surements of spontaneous microscopic thermally driven molecular concentra- tion fluctuations. Since its inception, and particularly in recent years, techni- cal and conceptual advances have extended the range of practical applicability and the information obtainable from FCS measurements. Improvements in microscopy, data acquisition, and data processing have greatly...