The underlying premise of many empirical, analytical, and numerical models of continental rifting is that mantle melt supply and/or temperatures prior to and during extension govern structural architecture and magmatic construction in the rift. However, drilling on the Iberia margin (Ocean Drilling Program [ODP] Legs 149 and 173) has documented extreme extension with little or no decompression melting of the asthenospheric mantle, defying model predictions. The puzzles raised by Iberia drilling are compounded by observations from geophysical studies on the conjugate Newfoundland margin, which document significant cross-rift asymmetries in basement depth, character of tectonic extension, and other deep structure. These results raise fundamental, overarching questions about rifting of non-volcanic margins, including the cause and extent of mantle unroofing, the presence or absence of decompression melting, the origin of the deep and crustal asymmetry between conjugates, the age-subsidence and strain-partitioning history, and the relation of rift events to development of shallow-water unconformities and the stratigraphic record. We propose three competing hypotheses for development of the Newfoundland–Iberia rift in which these questions are subsumed and which make specific predictions that can be tested by ocean drilling on the Newfoundland margin. The planned drilling is in a position exactly conjugate to the Iberia Abyssal Plain drilling transect and is based on a detailed grid of multichannel seismic and ocean borehole seismometer surveys that we conducted in 2000. The single most productive and direct test of our hypotheses can be accomplished by drilling a deep hole (~2200 m) in the central Newfoundland Basin (proposed Site NNB-01A), which is the primary target for drilling during ODP Leg 210. At this site we will also recover an extensive Cretaceous–Paleogene (and possibly Neogene) sedimentary record in the "gateway" between the Arctic/sub-Arctic seas and the main North Atlantic Basin. In the event that basement and deep sedimentary objectives cannot be achieved at proposed Site NNB-01A and sufficient time remains during the leg, alternate sites that extend seaward to known oceanic crust will be cored to partially meet our scientific objectives.