Seismic data, drilling data, and gravity data define a 40-km-wide continent-ocean transition (COT) below the outer half of the 70- to 80-km-wide shelf along the Leg 152 drilling transect. The seismic data include a detailed grid of shallow high-resolution seismic data; a regional grid of conventional multichannel seismic data (including sonobuoy refraction data); and a deep crustal, vertical incidence multichannel seismic profile. Below the outer shelf, the top of the Precambrian crust off southeast Greenland dips through a normally faulted, seaward-facing flexure zone below the landward feather edge of the seaward-dipping reflector sequence (SDRS), which is subaerially erupted baslatic lavas. Associated with the flexure zone, the continental crust thins from approximately 28 km thick to almost zero over a maximum distance of 40 km. Variations in present-day crustal thickness are much less because of accretion of thick igneous crust during breakup. Most of the thinning of the continental crust takes place within only 25 km from the mid-shelf flexure zone. Within this zone, landward-dipping normal faults are present in the upper continental crust, indicating that thinning was accommodated by fault-related crustal extension. Lower crustal thinning appears to be of a different nature and may be related to 'intrusive' underplating. The seismically defined COT is consistent with a distinct development from initially continental to oceanic volcanism observed in the boreholes. The new igneous SDRS crust seems to be 20-22 km thick, including at least 5 km of lava in the upper crust. Deep reflections located below the COT at 30-35 km depth may represent a pillow of thick (10 km or more) underplate dmaterial or intra-mantle reflections from a residual mantle.