This report discusses the reversal effect known as Simpson's paradox from a causal-theoretic viewpoint. It analyzes the reasons why the effect has been (and still is) considered paradoxical and why its resolution has been so late in coming. The report is extracted from a forthcoming book “”Causality”” [Pearl, 2000], and assumes some familiarity with causal diagrams and the do(.) (or sel(.)) notation (e.g., [Pearl, 1995]).