The heat flow in the northern Canadian Cordillera between 59 and 64 N is very high, 105 ¿ 22 mW/m2. The average upper crust heat generation is 4.6 ¿ 2.4 microW/m3, giving a reduced deep heat flow of 59 mW/m2, similar to the southern Canadian Cordillera, 49--53 N. Estimated Moho temperatures of 800--1000¿C and lithosphere only 50--60 km thick are substantiated by slow upper mantle seismic velocities and by high elevations associated with crust only 32--35 km thick. The new data support the conclusion that all of the North and South America Cordilleras are hot. The Paleoproterozoic Wopmay orogen to the east has a remarkably high heat flow of 90 ¿ 15 mW/m2 for such an old and stable region. However, basement samples indicate high heat generation and estimated deep temperatures are intermediate between the Cordillera and the Slave Craton further to the east. Heat flow from the Yellowknife area of the Slave Craton, 53 mW/m2, implies low lithosphere temperatures in agreement with kimberlite xenolith P-T data. The high temperatures in the Cordillera result in a very thin and weak lithosphere and may allow a lower crust detachment. The north and central Cordillera have been inland of the coastal Queen Charlotte transform fault since subduction stopped in the Eocene. Thus some process has maintained high temperatures in the former back arc for more than 40 Ma.