The results of a theoretical study of the interpretation of precursors to PP (delta approximately equals 100 deg) in terms of scattering by small-scale random irregularities in the crust and uppermost mantle are presented. These results are compared with observational data from two nuclear explosions recorded at the Warramunga array and three earthquakes recorded at the NORSAR array. This comparison reveals that the scattering interpretation offers a plausible and adequate explanation of the detailed observational data, namely: onset times, duration, slowness, azimuth and amplitude variations. The scattering interpretation of PP precursors, along with a similar interpretation of precursors to PKPPKP, removes the need for postulating sharp reflecting discontinuities in the uppermost few hundred kilometers of the crust and upper mantle.