  Dislocations are at the heart of the plastic behavior of crystalline
materials yet it is notoriously difficult to perform quantitative,
non-intrusive, measurements of their single or collective properties.
Dislocation density is a critical variable that determines dislocation
mobility, strength and ductility. On the one hand, individual dislocations can
be probed in detail with transmission electron microscopy. On the other hand,
their collective properties must be simulated numerically. Here we show that
ultrasound technology can be used to measure dislocation density. This
development rests on theory---a generalization of the Granato-L\"ucke theory
for the interaction of elastic waves with dislocations---and Resonant
Ultrasound Spectroscopy (RUS) measurements. The chosen material is aluminum, to
which different dislocation contents were induced through annealing and cold
rolling processes. The dislocation densities obtained with RUS compare
favorably with those inferred from X-ray diffraction, using the modified
Williamson-Hall method.
