Counterfactuals are routinely used in physical and biological sciences to develop and evaluate sophisticated, non-linear models. They have been used with telling effect in the study of economic history and American politics. For some historians, counterfactual arguments have no scholarly standing. They consider them flights of fancy, fun over a beer or two in the faculty club, but not the stuff of serious research. This dismissive attitude may be encouraged by the emergence and popularity of counterfactual historical works as a fictional genre, and the uncomfortable similarities between some recent works of counterfactual scholarship and such fiction. Nevertheless, counterfactuals are an effective research tool, but comprehending this requires a clear understanding of their nature, the circumstances to which they are best suited, and robust protocols for conducting thought experiments. With these ends in mind, the author begins this paper by exploring the differences between counterfactual and so-called "factual" arguments and offer the proposition that the difference between them is greatly exaggerated, it is one of degree, not of kind. Next he discusses the different uses of counterfactual thought experiments for historians. He distinguishes miracle and plausible world and scholarly and folk counterfactuals, and their respective uses, and proposes several criteria to guide plausible world counterfactuals. He concludes by looking at the special problems of applying counterfactual analysis to a problem as broad as the rise and success of the West. (Contains 41 notes.)