West Antarctica is generally thought to have behaved as a single terrane since mid-Cretaceous time. Subsequently some limited internal deformation, below the resolution of paleomagnetic methods, may have taken place, and there has been some extension between East and West Antarctica. However, the most dynamic part of the Antarctic plate during Late Cretaceous and Tertiary times was the Pacific margin of West Antarctica and adjacent oceanic areas. Unfortunately this is one of the least studied sectors of the Antarctic margin, due to its remoteness and inaccessibility. We present reconstructions of the Pacific margin of Antarctica based on constraints from marine magnetic data and regional free-air gravity fields. Recent results from interpretation of seismic reflection, seismic refraction and gravity profiles collected in the Amundsen and Bellingshausen seas are also incorporated. The reconstructions show regional constraints on the complex tectonic evolution of the Amundsen and Bellingshausen seas during and after the breakup between New Zealand and West Antarctica. The Late Cretaceous to early Tertiary development of the region involved interactions between the Pacific, West Antarctic, Phoenix (Aluk), Charcot and Bellingshausen plates. The West Antarctic margin consists of two main sectors. The western sector comprises the parts of the margin that Chatham Rise and Campbell Plateau separated from at about 90 Ma and 80 Ma, respectively. Part of this sector, probably including much of the Amundsen Sea continental shelf, was subsequently affected by independent motion of the Bellingshausen Plate. In the eastern sector, Phoenix Plate subduction continued through the Late Cretaceous and early Tertiary, then terminated progressively from southwest to northeast as segments of the AntarcticPhoenix Ridge migrated to the Antarctic Peninsula margin. Between these two sectors is an area with complex gravity anomalies near Peter I Island. We suggest that a fragment of the former Charcot Plate, containing the oldest ocean floor adjacent to West Antarctica, is preserved in this area.