We demonstrate that stability criteria can be used to calculate the maximum angle of stability u m of a granular medium composed of spherical particles in three dimensions and circular disks in two dimensions. The predicted angles are in good agreement with the experimental results. Furthermore, we determine the dependence of u m on cohesive forces, applying the results to wet granular material by calculating the dependence of u m on the liquid content of the material. We have also studied wet granular media experimentally and find good agreement between the theory and our experimental results.@S1063-651X~97!50512-5# Granular materials display a variety of behavior that distinguishes them from other forms of matter. Unlike solids, granular media conform to the shape of a container and will flow if the container is tilted sufficiently. Unlike liquids, however, a granular material is stable when its container is tilted slightly as long as the top surface is at a slope less than the angle of maximal stability u m . When the slope is increased above u m , grains begin to flow and an avalanche of particles occurs, the angle of the pile decreasing to the angle of repose u r . However, instead of uniform motion throughout the sample, all of the motion occurs in a relatively thin ~10 grains! boundary layer at the surface @1#. Experimental measurements of the angle of repose @2‐4# reveal that u r depends strongly on the shape and surface roughness of the grains. The typical measured value for u r is .22° for smooth spheres, butu r can attain 64° for materials containing rough, irregular particles. Cohesion between grains can also dramatically change the physical properties of a granular material, including u r and u m @5#. Such cohesion is commonly caused by the presence of a liquid in the material that forms interstitial bridges resulting in attractive forces between grains. While many experimental measurements of u r and u m have been made for different materials, few theoretical results are available regarding the numerical values of these angles. The most detailed theoretical predictions are provided by molecular dynamics studies @6‐8#, which have profoundly improved our understanding of u r , but have not provided a simple way to calculate u r or u m .