This chapter develops the rationale of causality. It is argued that causal models are regimented by a rationale of variation, not of regularity or invariance. Namely, causal models establish causal claims by evaluating suitable variations among variables of interest. It is also argued that regularity and invariance are constraints to impose on variations in order to guarantee their causal interpretation. Empirical, methodological, and philosophical arguments are offered. A taxonomy of variations is also sketched, and a thorough comparison between associational and causal models is offered in order to pinpoint the features of causal models guaranteeing the causal interpretation.