Graphical methods for investigating causality, the related [confounder] tag, do-calculus, interventions, and counterfactuals.
Causal diagrams provide a highly intuitive graphical method for investigating both interventions and counterfactuals. Perhaps most importantly, causal diagrams provide, finally, an unambiguous definition of a confounding variable: a confounding variable is a variable that sets up a backdoor path from the investigated cause to the investigated effect.
For more information, see The Book of Why, by Pearl and MacKenzie, Causal Inference in Statistics: A Primer, by Pearl, Glymour, and Jewell, and Causality: Models, Reasoning, and Inference, by Pearl.