In this chapter, we discuss one central property of electrolytes, ion solvation, which not only determines the bulk behaviors of an electrolyte such as solubility, viscosity and ion transport, but also affects interfacial and interphasial properties such as electrochemical stability and interphase chemistries. The preferential solvation of working ions plays a major role in dictating many key properties of electrolytes and subsequently their performance in electrochemical devices.