A eutectic mixture is a mixture of components that has a melting point below that of the pure components, and a deep eutectic solvent (DES) is a type of eutectic mixture that has a particularly deep depression of the melting point from that of the pure components. In a DES, the deep melting point depression is the result of strong interactions, such as hydrogen bonding and ionic interactions, between the components of the mixture. As liquids, DES have properties similar to ionic liquids, including low volatility, high thermal stability, and tunable physicochemical properties. Many DES are considered "green" solvents, because they are biodegradable, non-toxic, and inexpensive to synthesize. If the melting point is low enough, DES can be utilized as solvents for use in many chemical and industrial applications, including battery technologies, catalysis, separations, and waste processing. The components of a DES are labeled as a hydrogen bond acceptor (HBA) and a hydrogen bond donor (HBD), and are classified as type I-V based on the identity of the HBA and HBD. For example, type III DES have a quaternary ammonium salt as HBA, and the HBD is an organic molecule, type V DES have organic molecules as the HBA and HBD, and types I, II, and IV have metal chlordies. Many recent studies in the literature involve type III DES with choline chloride ((2-hydroxyethyl)-trimethylammonium chloride) or tetrabutylammonium chloride as the HBA, but there has also been an increase in the study of type V DES. Given the number of potential HBA and HBDs available, the possible binary (and ternary) DES combinations is extremely large, but there are currently no models capable of predicting what combinations will result in DES. This highlights the importance of exploring and characterizing new combinations of HBA and HBDs.