Mechanisms in a theory are defined here as bits of theory about entities at a different level (e.g., individuals) than the main entities being theorized about (e.g., groups), which serve to make the higher-level theory more supple, more accurate, or more general. The criterion for whether it is worthwhile to theorize at lower levels is whether it makes the theory at the higher levels better, not whether lower-level theorizing is philosophically necessary. The higher-level theory can be made better by mechanisms (...) known to be inadequate in the discipline dealing with the lower level. Conditions for the usefulness of lower-level theorizing are proposed, with many examples from various social and physical sciences. (shrink)