The theory of sputtering of amorphous and polycrystalline materials has been further developed. Experimental results for the energy, mass, and angular dependence of sputtering yields and recent experiments on the sputtering of chemi sorbed gas and sputtering by molecular ions illustrate sev eral effects that are important in determining sputtering yields and, in particular, the relationship of these effects to Sigmund theory. There are several mechanisms which might lead to discrepancies between theory and experiment.