There is growing interest in deep eutectic solvents (DES) for both chemical synthesis and materials applications. However, there is a general lack of understanding regarding what uniquely defines a system as a deep eutectic, and more importantly, what chemical properties determine unique eutectic behavior. In this work, we identify the thermodynamic basis for ideal eutectics relative to assumptions of ideal solutions. We then articulate the new Solvation Shell-Ionic Liquid (SSIL) and -Liquid Solvate (SSLS) model for solutions which identify the chemical/physical factors that create solution non-ideality and afford the ability to accurately calculate liquiduses, and thus the eutectics, of binary systems. The application of these models is demonstrated using diverse families of materials including aqueous salt solutions (ZnCl 2 , NaCl, KCl, and NH 4 NO 3 ), aqueous molecular solutions (sucrose and urea), solutions of naphthalene with aromatic (toluene, benzene, toluene, biphenyl and phenanthrene) and non-aromatic (ethanol, butanol, heptane and acetone) solvents, and the choline chloride:urea system for which the term DES was first introduced. The accurate description of the non-ideal enthalpic and entropic terms affords clear definition and prediction of both deep and elevated eutectics in diverse systems.