The primary aimof this paper is to analyze the concept of degrees of causal contribution for actual events and examine the way inwhich it can be formally defined. This should go some way to filling out a gap in the legal and philosophical literature on causation. By adopting the conception of a cause as a necessary element of a suffi- cient set (the so-calledNESS test) we showthat the concept of degrees of causation can be given clear and even empirical meaning. We then apply a game theoretical framework to derive a measure of causal contribution. Our favouredmeasure turns out to be a generalised version of the normalized Penrose–Banzhaf index of voting power.