We formulate an analogy between thermodynamically stable water/oil microemulsions and surfactant-stabilized nanocrystal dispersions, with the goal of accounting for how specific sizes and shapes of nanocrystals might become dominant in equilibrated solutions of, for example, alkali halides or semiconductors in organic solvents. Qualitative estimates of the energetics-i.e., of the dependence of lattice energy on crystal dimensions-suggest that the size and shape distributions can be quite narrow, with their average properties controlled primarily by the solution composition, in particular the ratio of surfactant to crystalline species.