Repeated eruptive and intrusive activity on the East Rift Zone of Kilauea volcano allows dikes to exist within the rift zone for extended periods of time. Leakage of magma from the summit storage region into the rift zone can cause inflation of a rift zone dike. Inflation raises the excess pressure at the dike centre and increases the stress intensity at the dike edges. If the inflation is sufficiently large, the stress intensity may exceed the fracture toughness of the country rocks and allow vertical or lateral dike emplacement to occur. Theoretical modelling can be used to predict the conditions under which vertical or lateral dike propagation and hence eruptive or intrusive activity may occur. Over typical ranges of dike half height and regional tension gradient, vertical propagation and hence eruption are favored by high fracture toughness, relatively low dike half heights, and low regional tension gradients. Lateral propagation can occur when the fracture toughness is low, when dike half height is higher, and when the regional tension gradient is high. The results of from the model constrain the fracture toughness within the East Rift Zone to be 100--110 MPa m1/2 and the regional tension gradient to be ≪1000 Pa/m. ¿ American Geophysical Union 1991