Evidence favoring an impact origin for the Sudbury Structure continues to accumulate, particularly the recent discovery in the Black Member of the Onaping Formation of shocked microdiamonds resembling those found at other impact structures. The 1990 Lithoprobe Vibroseis survey of the Sudbury basin and its subsequent interpretation indicated that the original diameter of the Sudbury Structure was of the order of 200 km, as opposed to previous estimates of 60-100 km. This new size estimate made it possible to interpret the Sudbury Igneous Complex (SIC) as an impact melt with no requirement for a mantle-derived contribution.The very felsic compositions of all of the exposed components of the SIC make it difficult to account for the origin of the dunite, harzburgite, wehrlite, websterite, and melanorite inclusions in the Sublayer at Sudbury, without calling upon their derivation from a preexisting layered intrusion in the target area. Recent U-Pb dating of baddeleyite and zircon from melanorite inclusions, giving a 1.85-Ga age, indicated that these are related to the development of the SIC, and are not from an older intrusion. However, new Re-Os isotopic data on variably mineralized melanorite inclusions indicate that the ore component of these inclusions has interacted with a dominantly crustal source with an average age of about 2.6 Ga, while the silicate component is younger. The solution to this enigma is probably that the mafic/ultramafic inclusion suite is composite, the more olivine-rich inclusions being from a preexisting source and the melanorites being of Sudbury age, with a major component of the mineralization also predating the SIC.Modeling the fractional crystallization of hypothetical SIC magmas indicates that the rock types, and the sequence of cumulus phases, is consistent with a fractionation origin, but that the relative proportions of the rock types is not; there is a great excess of granophyre relative to norite exposed at surfrace. The application of computer models designed to simulate the impact of large bolides to the Sudbury Structure leads to the conclusion that the resulting impact melt would have been superheated for the first 100,000-200,000 yr of its existence, as it cooled from about 1,700°C to its liquidus temperature (about 1,200°C). During this time a massive amount of the overlying basal breccia of the Onaping Formation would have been melted to accumulate at the top of the underlying impact melt. Sulfur solubility in silicates melts is strongly temperature-dependent, so that cooling through this 500°C temperature interval of superheat would also have favored liquation of a sulfide liquid if the initial impact melt had been anywhere close to sulfide saturation, and the very complete settling of this liquid to the base of the complex.It is proposed that the bolide impacted an area of the crust underlain in part by a preexisting mafic intrusion, and that the initial melt was more mafic than average Naldrett, A.