In this review and discussion of future opportunities for research, we will explain the mechanical behavior of hexagonal close packed (HCP) metals and alloys, mainly Mg- and Ti-based, which have been subject to severe plastic deformation (SPD) processing. There will be four major points of emphasis, as the behavior of these materials is compared and contrasted with that of face centered (FCC) and body centered (BCC) cubic metals and alloys. First, given the venue, there will be specific attention played to the behavior at high strain rates. Second, the greater sensitivity to crystallographic texture will be emphasized, as it can sometimes overwhelm the grain size effects common to SPD processed material. Third, the propensity for mechanical twinning, especially at high strain rates, will prove important since it can induce a degree of asymmetric behavior (tension/compression strength differential) which is rare in FCC and BCC metals. Even Ti-6Al-4V, which does not typically exhibit significant twinning, does so at dynamic strain rates. Finally, given the variety of slip and twinning systems that may be activated, each with their own thermally activated behavior, unique issues of strain rate sensitivity will be discussed. It will be shown that, although a great deal has been learned over the last 20 years, there remain outstanding questions and leveraging opportunities, including hints to help understand the mechanical behavior of additively manufactured materials.