Strain rate sensitivity (SRS) is a material property that measures how the flow stress in a material changes with the strain rate. Among common metals, magnesium (Mg) exhibits a high SRS value, and the SRS value varies significantly with the alloy compositions and the loading directions. In this work, the above issue is studied by examining how the activation of each slip mode is affected by the strain rate. To this end, in-situ three-dimensional X-ray diffraction (3DXRD) was employed to investigate the tensile deformation of a Mg–3wt.%Al alloy at strain rates of 10-4, 10-3, and 10-2 s-1, respectively. 3DXRD enables the characterization of more than 500 grains in each sample. From the observed grain orientation rotation and the broadening of the associated diffraction peaks, slip system activation can be identified in individual grains; from the grain-specific stress tensor, the critical resolved shear stress (CRSS) of the activated slip system can be determined. CRSS for basal, prismatic, and pyramidal <a> slips all increase with the strain rate. SRS of the three slip modes were estimated to be 0.0036, 0.0226, and 0.0106, respectively. Basal slip being less strain rate sensitive than non-basal slips can explain the texture dependence of SRS in wrought Mg alloys. Additionally, we have recently conducted in-situ 3DXRD experiments on magnesium alloys with various microalloying elements. Aiming to explore the influence of different alloying elements on the activation of various slip systems and their effects on the mechanical behavior of magnesium alloys.