Mg and Ti alloys are attractive materials for structural applications in the transportation and biomedical industries due to their high strength-to-weight ratios. However, due to their hexagonally close-packed (HCP) lattice structure, they exhibit poor formability at room temperature and strong plastic anisotropy. The anisotropy often results in non-intuitive deformation behaviors. In situ test methods such as x-ray diffraction, neutron diffraction, and high-resolution digital image correlation are powerful tools that make it possible to examine the evolution of the microstructure during deformation. In this thesis, the plastic deformation behavior of three HCP metals is examined using various in situ mechanical test techniques. Each study focuses on a particular aspect of the plastic deformation: the effect of strain path changes on Mg AZ31B, the high work hardening of a Ti-6Al-4V alloy, and the three-stage work hardening and springback of a Grade 2 Ti.