Debris flow is the gravity-propelled movement of sediment-water mixtures, in which grains are supported above the non-moving bed by the strength and buoyancy of a clay-water matrix. The competence (largest supported grain size) of a debris flow is thereby determined by the magnitudes of matrix strength and density, which vary primarily and directly with matrix clay content. Strength also varies with the type of clay mineral and with the cation content of the water. Experiments reported here indicate that as little as 1.5 to 4 wt. % clay mixed into water is sufficient to support finest sand. The experiments also show that competence is less in flowing debris than in static debris, and that competence decreases with flow duration for durations less than about one hour but remains essentially constant for durations greater than one hour. Competence apparently is independent of flow velocity or velocity gradient, although low velocities and gradients were not examined here. Calculations based on the experimental data reveal that marine sediments with finest sand as their coarsest material and with bulk clay contents as low as 2 wt. %, or less, can move as debris flows. Sediments with coarsest sand can move as debris flows if they contain as little as 19% clay, or less. Considering that these values are high estimates, many finegrained marine sediments have clay contents great enough in order to move as debris flows. Zones of various competences should exist within a debris flow because of the nature of the velocity profile and the dependence of competence on shear and flow duration. These zones are preserved during deposition and therefore, debris-flow deposits expectably should have layers of various grain sizes, and perhaps be normally and/or inversely graded. Debris flow is discussed in this paper as an idealized concept, where grain support is wholly by strength and buoyancy. In real flows, support also may be provided by graininteraction dispersive pressure and by turbulence, increasing the competence of the flows. Debris-flow mobility, in terms of the forces required to keep the flow moving, was not considered here but is equally significant to competence in evaluating the occurrence of real debris flows.