In the front material of this book both a foreword and a preface appear. What the content of a preface should be is well understood. It is the author's retrospective account of intent, of the labors to accomplish that intent, and of the content of the book that resulted. What a foreword should be is less obvious. Most properly, it is perhaps the brief testimony of one who knows the accomplishments of the author and the scope of the field and who may direct readers to the book. On some basis, the writer is assumed to have earned the right to undertake such a task. To undertake the writing of a foreword for so considerable a researcher, teacher, and scholar as Alfred Evans can be seen not only as an honor but also as a daunting one. My first thought, in truth, is that this wine needs no blush and that no foreword is needed. As John Rodman Paul Professor of Epidemiology at Yale, Alfred Evans has an established reputation in the field of causality. We have learned from his insights about the evolution of causal thinking as epidemiology passed from the era of the germ theory into that of the search for causes of chronic noncontagious diseases. It was he who drew attention to the effect of specific context in that evolution.