A comprehensive study of the mineralogy of the famous Sudbury ores [Ontario] is applied to the problem of their origin. A critical review of the many confusing and controversial geological problems of the region, however, has been required to establish premises on which a theory can be soundly based. Following this, the actual study is presented covering the nature of the ores, their detailed mineralogy and chemistry, microscopic relations of ore to rock-forming minerals, and distribution of metals and minerals. An interpretation of the history and origin of the ores, based on both geological and mineralogical evidence, is then given. What may be termed the older classical view of the geological history of the area, after Coleman, Collins, and Sudbury field geologists, with minor modifications, is favored. The ore-bearing Ni irruptive, clearly pre-Keweenawan in age, is considered as having been intruded as a single body much in the form it now shows, following, for the most part, a pronounced unconformity, pre-Animikean(?) in age, below which are highly altered, granitized, and brecciated pre-Huronian volcanics and sediments with rare remnants of lower Huronian sediments unconformably above them. Extensive brecciation during extreme diatreme activity is considered as a prelude to the unique volcanic activity giving rise to the lower part of the Whitewater Series, within the Sudbury basin, the Onaping tuffs and breccias (glowing avalanche deposits) and associated sediments which lie above the irruptive, are altered by it, and structurally appear unconformable to formations below. An Animikean rather than early Precambrian age for the Whitewater is suggested. Postirruptive events include 2 or more orogenies (Penokean and Grenville), a period of faulting and fracturing and late Pb-Zn mineralization, minor intrusion of postnorite granite, and later (Keweenawan) trap and diabase, events locally modifying the ores. Various classifications of ore types are given including a new one, named an "immiscible-silicate-sulfide" ore in which silicate blebs are dispersed in sulfides, the counterpart of some disseminated ores in quartz diorite or norite. Detailed descriptions of over 60 metallic and non- metallic minerals are given showing chemical variations, trace-element content and textural relations. Many metallic minerals (sulfides, arsenides and others) are recognized as of different generations, and many participate in ex-solution intergrowths as well as peculiar pseudoeutectic textures. Available data on distribution of the common metals, Pt group metals and minerals, Au and Ag, Co and Se, as well as Bi and others are given, and rare but important instances of zoning are noted. Interpretation of the complex history of the ores is based on observed textural relations of the minerals, studies of synthetic sulfides and arsenides, and on the similarity of textures produced synthetically to those found in the ores. Comparisons are also drawn with other natural occurrences and with the mode of crystallization of sulfide-oxide-silicate systems showing liquid immiscibility.