Fluorescence quenching refers to any process which decreases the fluorescence intensity of a given substance. A variety of processes can result in quenching. These include excited state reactions, energy transfer, complex formation, and collisional quenching. In this chapter we will be concerned primarily with quenching resulting from collisional encounters between the fluorophore and quencher, which is called collisional or dynamic quenching. We will also consider static quenching, which is due to complex formation. Static quenching is a frequent complicating factor in the analysis of dynamic quenching. In addition to the processes described above, apparent quenching can occur due to the optical properties of the sample. For example, high optical densities or turbidity can result in decreased fluorescence intensities. This is a trivial type of quenching which contains little molecular information. Throughout this chapter we will assume such trivial effects are not the cause of the observed decreases in fluorescence intensity.