A review on the microstructural evolution in magnesium alloys during severe plastic deformation waspresented. The challenges deserved to achieve ultrafine/ nanostructured magnesium were discussed.The characteristics of the processed materials are influenced by three main factors, including i)difficult processing at low temperatures, ii) high temperature processing and the occurrence ofdynamic recrystallization and grain growth processes, and iii) a combined effect of grain refinementand crystallographic texture changes. Reviewing the published results indicate that there are twopotential difficulties with severe deformation of magnesium alloys. First, it is very hard to achievehomogeneous ultrafined microstructure with initial coarse grains. The second is the dependency ofmicrostructure development on the initial grain size and on the imposed strain level. It was clarifiedthat different grain refining mechanisms may be contributed along the course of multi-pass severedeformation. It was clarified that discontinuous recrystallization takes places during the first stages ofdeformation, whereas continuous refinement of the recrystallized grain may be realized at consecutivepasses. Shear band formation as well as twinning were demonstrated to play a significant role in grainrefinement of magnesium alloy. Also, the higher the processing temperature employed the morehomogeneous microstructure may be achieved with higher share of low angle grain boundaries.