The new collection of essays edited by John A. Vasquez in What Do We Know About War? provides a useful overview of the quantitative literature on war. This book makes no claims to move the field forward significantly but, instead, offers seniors and first-year graduate students a good basic understanding of how statistical analyses have been used to explain the variation between war and peace. The book has significant strengths and weaknesses. Its strengths are the wide array of questions addressed and the attempt to provide a systematic discussion of the current state of the quantitative knowledge on war; its weaknesses are the paucity of attention paid to new insights from the rational choice literature and their implications for the quantitative study of conflict.