Large rockfalls and debris avalanches constitute spectacular geologic hazards. A physical basis for the prediction of the extent of runout of such transport events has remained elusive. We consider the simplest case in which a mass M of debris and loose rock, having fallen from a height H, is subjected to a constant, overall resisting shear stress τ during runout. A prediction for such behavior is that the area overrun by an avalanche is proportional to (gMH/τ)2/3, where the coefficient of proportionality is near unity and a function of the geometry of the “footprint” of the avalanche deposit. This scaling results in a good collapse of the data for a wide range of terrestrial and extraterrestrial phenomena and implies a value of τ in the range 10–100 kPa. Such shear stress values are comparable to measures of the yield strength of unconfined, dry debris obtained by other means. The approach developed here does not give a detailed description of rockfall motion, but provides new insight for attempts to delineate the mechanisms that contribute to the mobility of rockfalls and other densely concentrated flows of geophysical interest.