This chapter introduces the concept and principle of "quantum causality," and makes suggestions toward rethinking the nature of causality in view of the possibility of such a concept and principle, in juxtaposition to the classical concept and principle of causality, as defined in Chap. 1. Sec. 5.1 serves as a general introduction. Sec. 5.2 offers a discussion of the classical philosophical understanding of causality. Sec. 5.3 considers classical physics and relativity as classically causal theories, with a particular emphasis on qualifications that the use of classical causality in these theories requires. Sec. 5.4 considers quantum mechanics, in nonrealist, RWR-principle-based, interpretations, which preclude classical causality, and proposes the concept and the principle of "quantum causality."