Recent seismological studies suggest that slab detachment has occurred in the Mediterranean and the New Hebrides subduction zones. Subducted slabs in these regions are recognized to be torn at depths ranging from 100 to 300 km, presumably caused by the lateral migration of the tear along the strike of the slab. To investigate the physical mechanism of the slab detachment and in particular its migration, we constructed a viscoelastic three-dimensional finite element model and introduced a small initial tear from one side of the slab. We investigated spatio-temporal variations in the state of the stress within the slab, as a function of tear length, rheology, and a variety of force distribution. Our results show that an area of high shear stress concentration of the order of several hundred magapascals forms near the tip of the tear inside the slab, which is probably sufficient to cause further lateral migration of the tear. The stress concentration increases with the length of the tear and lower viscosity values of the surrounding mantle and increases with downdip tension. From our modeling, we conclude that favorable conditions for slab detachment are characterized by a high interplate frictional force at a subduction zone and a low convergence rate, forming in-plate tensional stress at intermediate depths. Such a condition is indeed observed in the Dinarides/Hellenic and the New Hebrides subduction zone. ¿ American Geophysical Union 1995.