Rationalist, psychological, and domestic politics approaches have all generated internally consistent, plausible explanations for long wars. But sorting out which of these explanations is most valid is quite difficult, because definitional questions bedevil the study of war duration, and more importantly, because it is very hard to evaluate the evidence for competing explanations of war duration. The latter difficulty arises for three reasons. First, many state behaviors are consistent with multiple, competing explanations of long wars. Second, in most states, multiple people play important roles in crafting foreign policies, meaning different leaders may have different primary motives for continuing a war. Last, even individual leaders may be driven by multiple motives. These challenges are relevant for both case study research and large-N studies. Recognizing these challenges should help to improve future studies of the causes of long wars.