The relative merits of methods of determining the density of dislocations are reviewed. In particular, consideration has been given to the evidence available tending to indicate a loss of dislocations from foils of deformed metal during thinning processes preparatory to viewing in the electron microscope. By measurement of the electrical resistivity ratio, 293˚K./77˚K., during thinning, annealed specimens of super-purity aluminum foil were shown to obey Fuch’s theory for thin films, but cold-rolled specimens could not be fitted to the theory. This discrepancy has been attributed to the loss of dislocations from the cold-rolled material, and the results are shown to be consistent with a “loss-fringe” model of dislocation loss from the surface. / Thesis / Master of Science (MSc)