The Lomnitz and Futterman theories of internal friction are shown to be so similar that experimental distinction between the two is unlikely at present. The Futterman theory is largely ad hoc, and for this reason the Lomnitz theory should be preferred. Both theories require that a measurable amount of dispersion accompany attenuation. In fact, the detection of the proper amount of dispersion is crucial in demonstrating that any linear theory can explain the observed attenuation. Wuenschel has detected dispersion of the correct amount in in situ experiments in the Pierre shale and laboratory experiments in Plexiglas, but O'Brien and Lucas have failed to detect any significant dispersion in a statistical compilation of a large number of oil well logs.