Cryomilling is a method of producing nanostructured morphologies from a range of starting metals and alloys. This process substantially increases the strength of lightweight alloys. In this work, we characterize the microstructure of Al alloy 5083 following cryomilling, hot isostatic pressing (HIP), and extrusion. The yield strength of the cryomilled 5083 Al is approximately twice that of conventional wrought 5083 Al and the room temperature microhardness essentially is unchanged following annealing at temperatures that approach 0.8 Tm. Using complementary transmission electron microscopy (TEM) techniques such as energy filtered (EFTEM) and weak beam imaging we investigate the mechanisms responsible for the mechanical properties. A survey of the microstructure identifies several sources of strengthening. These include: submicron grain sizes in the as-extruded material, precipitates, and metal-oxide phases. Also, the Mg in the alloy is expected to contribute some solid solution strengthening. TEM images show that lattice dislocations frequently are pinned at the precipitate interfaces. Continued precipitation and grain boundary pinning by oxide particles at elevated temperatures may account for the persistence of hardness following annealing.