Preventing cancer depends on the ability to recognize and remove causal factors. In current practice, the methods used to judge cause from epidemiologic, clinical trials and biologic evidence include systematic narrative reviews, criteria-based inference methods, and meta-analysis. Subjectivity and values play a key role in the practice of causal inference, especially in selecting criteria and assigning rules of evidence to those criteria. Judging cause is a central concern of physicians, epidemiologists, and other public health professionals committed to cancer prevention.