We examine the interaction of seismic waves with heterogeneity at all scales, with an emphasis on the influence of structure on multiple scales.Strong interactions occur when seismic wavelengths are comparable to the size of heterogeneity, producing complex scattering. Because seismic waves span a broad spectrum of frequencies and hence wavelengths, any heterogeneous structures will be perceived in different ways by the various aspects of the wavefield, with significant difference in behaviour between body waves and longer period surface waves. Such complications become most evident when a wide range of heterogeneity scales arepresent simultaneously, as in the lithosphere.