The effects of a supersonic transverse gas flow over an opaque solid surface irradiated by a high−power cw laser is investigated. The motion of the gas parallel to the laser−induced melt layer produces two effects. The first is to feed energy into unstable surface disturbances. These instabilities do not grow indefinitely but become steady periodic waves. The second is the exertion of a shear stress at the gas/melt interface which in turn establishes a velocity profile in the melt. It is this second effect which results in melt removal at the trailing edge of the melt region. This removal increases the recession rate of the melt/solid boundary propagating through the solid. It is this shear−stress removal mechanism that is investigated. The analysis considers both gas and liquid viscosity.