This book sets forth a tentative effort toward the production of a scientitic treatise on mission theory from the Catholic point of view. No pretension, however, is made to offer an exhaustive work: the author would present merely a sketch of something to be looked forward to in fullest detail, presumably from other hands, at a later date. But the production of such a complete work as here is suggested would be impossible of accomplishment at the present stage of the progress of research toward establishing an adequate basis for mission theory Let it be understood, then, from the start, that this' volume purports to be only the beginning of a va.st work which certainly should and must be furthered and completed, in another connection, in the future. The text is in essence merely a somewhat amplified version of a course of lectures delivered at Muenster University, rearranged considerably and prepared for publication. The schematic endeavor is rather that of one making a first furrow in an uncultivated field, thus offering an incentive and encouragement for others to help break up the rough soil and proceed to intensive cultivation and practical and scientific utilization of the reclaimed territory. All this is merely to say that perhaps others, even now working here and there in patches of the same ground, may gradually come together in the realization of a thoroughly worked-over and prepared area, which may later be made of .avail by one, or all, or by some other, taking advantage of these advance labors to finally bring forth the ultimate work toward which all earlier experiments are always a necessary and inevitable contribution. As the reader will observe for himself, the author has endeavored, in so far as opportunities for present research and materials would allow, to present the problem in its main outlines by means of the building up of a tentative theoretical structure on empirical grounds. To this end he has taken the utmost advantage of such positive and practical regulations as have been formulated from time to time in the history of the Church by the ecclesiastical and missionary authorities, although he is forced, on this very count, to ask the indulgence of the reader, for the simple reason that it was found impossible to get together sufficient material of this kind to fill all the gaps and enable one to build up a complete fabric on this side of the whole question: many desired documents were not to be found, use of many others was positively denied on the ground that their publication would not be suitable in this connection; and again, it must be conceded to be utterly beyond the powers of anyone worker in this field at the present day to search out and bring to light all needed sources of this kind; or to exhaustively study all that come to hand, and so to be ultimately prepared to draw from them a thoroughgoing and satisfactory explication of mission law from this background.