We investigate the multiscale micromechanical behavior of nearly pure polycrystalline aluminum exhibiting randomly oriented coarse grains (ca. 300 m in size) between room temperature and 400°C. We present results from in situ mechanical testing obtained through scanning electron microscopy and full-field strain measurements by digital image correlation (DIC) during uniaxial compression, with controlled displacement rate. Direct observation of the process of developing strain heterogeneities allows for identification of the active mechanisms, characterization of their interactions and quantification of their respective contributions to the overall strain. The full-field strain measurements were carried out, from the sample scale, to the scales of the aggregate of grains, and finally the single grain. DIC analysis was performed thanks to specific surface marking patterns obtained by electron microlithography appropriate for the different scales of interest. The strain localization patterns showed dominant crystal plasticity. Except at room temperature, we always observed simultaneous and continuous activity of grain boundary sliding, whose relative contribution increased with temperature. We suggest that for coarse-grained microstructures grain boundary sliding acts as a complementary mechanism for the accommodation of local plastic incompatibilities inherent to the anisotropy of crystal plasticity.