The focus of this chapter is on liquid solutions in which one component is present in much greater abundance, say at least one order of magnitude greater, than all others. Examples that underscore the importance of this type of solutions include seawater, and natural terrestrial waters in general, but one can imagine more exotic possibilities, such as hydrocarbon-based solutions on Titan's surface and ammonia-based solutions in its interior. What all of these examples share is the fact that it is convenient to make a distinction between dilute solutes that may not be liquid in their standard states, and a liquid solvent that is generally close to being in its standard state. Depending on the nature of the solvent and of the solutes the latter may exist as electrically neutral chemical species, as ions, or as a combination of both. Solutions in which solutes dissociate into ions are known as electrolyte solutions. Among these, those in which water is the solvent are by far the most important ones, at least in terrestrial environments. The chapter emphasizes aqueous electrolyte solutions, but virtually all of the thermodynamic framework is applicable to any type of dilute solution. We begin with a discussion of dilute solutions in general, and shift the focus to electrolyte solutions in Section 11.3.