Deliquescence is a first order phase transition from solid to solution that occurs at a relative humidity (RH) that is characteristic to the crystalline compound. Such dissolution of active pharmaceutical ingredients and excipients can lead to detrimental physical and chemical instabilities. Furthermore, in systems containing more than one deliquescent component, the RH of the solid-solution transition will be lowered, leading to some level of dissolution at unexpectedly low RH conditions. Deliquescence lowering is independent of the ratio of the deliquescent components and therefore is of concern for any formulation containing two or more deliquescent compounds. Because chemical reactions occur much more readily in solution, deliquescence will enhance the degradation of labile APIs. RH fluctuations will lead to cycles of deliquescence and efflorescence (crystallization), which will contribute to particle agglomeration and caking. This review will address the phenomenon of deliquescence, the significance of deliquescence to pharmaceutical systems, measurement techniques, the kinetics and thermodynamics of deliquescence, the behavior of mixtures of deliquescent compounds (including phase diagrams and thermodynamics of binary systems), and consequences of deliquescence on chemical and physical stability.