Publisher Summary This chapter discusses the fundamental physics of fluorescence. The application of fluorescence to microscopy represents an important transition in the development of microscopy, particularly as it applies to biology. It enables quantitating the amounts of specific molecules within a cell, determining whether molecules are complexing on a molecular level, measuring changes in ionic concentrations within cells and organelles, and measuring molecular dynamics. The chapter also discusses the issues important to quantitative measurement of fluorescence and focuses on four of quantitative measurements of fluorescence—boxcar-gated detection, streak cameras, photon correlation, and phase modulation. Although quantitative measurement presents many pitfalls to the beginner, it also presents significant opportunities to one skilled in the art. The chapter also examines how fluorescence is measured in the steady state and time-domain and how fluorescence is applied in the modern epifluorescence microscope.