The polarity of a solvent is a fundamental determinant of its use in chemistry. For molecular liquids, the principles that explain the relationship between molecular structure and solvent polarity are sufficiently well-understood to guide experimentalists in the proper choice of solvent. These principles are vastly different for ionic liquids. We present the results of theoretical modeling on ionic liquids that clarify this relationship, and express simple principles that researchers can use to select the appropriate ionic liquid for a given task.