Causation is an asymmetric relation - if C causes E, then E does not cause C. In this book it is argued that: 1) Two major theories of causation - the regularity theory and the counterfactual theory - cannot adequately account for causal asymmetry; 2) Causal asymmetry consists in the explanatory asymmetry between cause and effect; 3) Generally, the notion of causation is dependent on the notion of explanation; in other words, explanation sets a conceptual constraint on causation. In reaching these conclusions, issues about simultaneous causation, backwards causation and absence causation are discussed.