Simple examples of several common hydrothermal flow regimes have been found on the eastern flank of the Juan de Fuca Ridge. Ocean Drilling Program Leg 168 will focus on two of these, at sites where (1) there is a sharp transition from sediment-free to sediment-covered igneous crust, where heat flow, crustal temperatures, basement fluid compositions, and upper crustal seismic velocities all show large systematic changes associated with the transition from open to closed hydrothermal circulation, and where (2) buoyancy-driven fluid flow within the igneous crust and through the seafloor is strongly influenced by basement relief and sediment-thickness variations. If time permits, a third site may be studied where particularly flat basement and regionally continuous sediment cover should prevent advective heat loss and allow the total heat flow from this young lithosphere to be determined confidently. All of these examples represent situations that occur commonly in all ocean basins, but because of the unusual simplicity of the occurrences in this area, they provide ideal targets for drilling. Sampling, downhole measurements, and postdrilling observations will be directed mainly toward elucidating the physics and fluid chemistry of ridge-flank hydrothermal circulation, and the consequent alteration of the upper igneous crust and sediments that host the flow. Two arrays of relatively shallow holes will provide direct information about lateral gradients in basement fluid composition, formation pressures, and temperatures and strong constraints on formation-scale heat-transport properties of the upper igneous crust and the vigor of circulation.