This book presents the theoretical background to fluorescence correlation spectroscopy (FCS) and a variety of applications in various fields of science. FCS is based on the detection of single molecules excited to fluorescence in diffraction limited confocal volume elements and the time correlation of stochastic events. It provides ultimate sensitivity in the analysis of molecular processes and has found numerous applications in physics, chemistry and particularly in biomolecular sciences. Its high spatial and temporal resolution has made FCS a powerful tool for the analysis of molecular interactions and kinetics, transport properties due to thermal motion and flow, as well as the physics of the excited state in solution as well as at the cellular level. Its application in high throughput drug screening is using all the potential of this prime analytical tool.