Natural gas hydrate occurs beneath many continental slopes and in arctic permafrost areas. Recent studies have indicated that the largest deposits of gas hydrate might lie in nearly horizontal layers several hundred meters beneath the seafloor of continental slopes, especially in the large, accretionary sedimentary prisms of subduction zones. Expedition 311 of the Integrated Ocean Drilling Program (IODP) investigated the formation of gas hydrate in the accretionary prism of the Cascadia subduction zone (Fig. 1). The primary objectives of Expedition 311 were to test and constraingeological models of gas hydrate formation by upward fluidand methane transport in accretionary prisms. We specifi -cally sought to (a) determine the mechanisms that controlthe nature, magnitude, and distribution of the gas hydrate,(b) find the pathways of the fluid migration required to formlarge concentrations of gas hydrate, (c) examine the effectsof gas hydrate on the physical properties of the host sediment,and (d) investigate the microbiology and geochemistryassociated with the occurrence of gas hydrate. Furthermore,we concentrated on the contrast between methane transportby focused fl ow in fault zones and by dispersed pervasiveupward flow at various scales of permeability.