We critically review concepts of self-organization and self-assembly, and extract from this analysis consistent and workable definitions of both concepts that are applicable across many scientific disciplines. In essence, we distinguish self-organization from self-assembly on a thermodynamic basis, where self-organization implies a nonequilibrium process and self-assembly is reserved for spontaneous processes tending toward equilibrium. This discrimination is consistent with early uses of both terms. © 2008 Wiley Periodicals, Inc. Complexity, 2008.