THE EVIDENT confusion and contradictions that have marked references to the nationality of merchant vessels by jurists, publicists, and others have prompted this study. It was apparent that there existed no clear-cut conception of exactly what state of facts created such a relationship between a vessel and a State. Particularly in time of war the concept of the nationality of a vessel has been dealt with so loosely as to quite obliterate the exactness of its meaning. Yet that there must be a precise meaning in international law for a term of such historical and universal usage seemed reasonable. It was thought that a careful survey of the acts and official declarations of States, when confronted with problems involving the nationality of ships, would unearth some factual and definite basis upon which this relationship rests. Such has actually been the case, and the evidence is presented herewith. It would not be seemly to leave unacknowledged the help of those whose unfailing and amiable cooperation have contributed so much toward lightening the burden of the work. To the Graduate Faculty in Political Science at Ohio State University the author is particularly indebted for the grant of a fellowship making it possible for him to devote his entire time to research. For their faith in the value of this undertaking the author is therefore grateful to Dean William McPherson and Professor Henry Spencer, and especially to Professor Harvey Walker of that institution.