This chapter systematically explores the notion of invariance, which is the key feature that a generalization must possess if it is to figure in causal explanations. A generalization is invariant if it is stable in the right way under interventions. A generalization may be invariant even if it fails to satisfy such traditional criteria for lawfulness as exceptionlessness, expressability in ter m s of purely qualitative predicates, and wide scope. Laws are one kind of invariant relationship but not the only kind. Invariance is also different from Skyrms’ notion of resiliency, and satisfaction of the Mill-Ramsey-Lewis criteria for lawfulness is neither necessary nor sufficient for invariance.