A remarkable characteristic of the surface of Phobos is that it is densely covered by grooves-long linear depressions formed in a regolith. Typically, grooves are 100--200 m wide, 10--20 m deep, and have smooth concave-up cross profiles. Some can be traced for up to 30 km. Groove paths follow the intersection of planes with the surface of Phobos. Groove planes are grouped in four sets of parallel members. All sets are nearly parallel to the intermediate axis of Phobos: one is parallel to the equatorial plane, another perpendicular to the long axis of Phobos, the other two sets make symmetric angles (~25¿) with the equatorial plane. Grooves are widest and deepest near Stickney, the largest crater on Phobos, and are absent from the trailing region of Phobos, roughly opposite to Stickney. Groove formation occurred during a restricted time interval, which, on the basis of superposed craters, closely followed the formation of Stickney. The impact that formed Stickney probably opened fractures along preexisting planes of weakness and caused subsequent formation of the grooves either by drainage of regolith into open fractures or by ejection of regolith overlying the fractures or a combination of both oprocesses. Heating due to the impact may have produced enough steam from the satellite's carbonaceous chondrite material to expell regolith along the fractures. Other proposed mechanisms of groove formation, such as tidal stressing, internal heating, capture fragmentation, etc., cannot account adequately for the pattern, age, and morphology of the grooves. The lack of grooves on Deimos could be explained by the absence on its surface of a crater large enough to have severely fractured the outer satellite.