Improvement of ductility is important for applications of Mg alloys. Basal dislocation motion and twinning are the two major deformation modes in Mg alloys. However, the basal slip system cannot support homogeneous plastic deformation of Mg alloys. Twin-boundaries in Mg alloys are potential cracking sites. Therefore, activation of non-basal slip is expected to play an important role in improving ductility of Mg alloys. We have studied both <a> and <c+a> dislocations, as well as their interactions with solute atoms, stacking-faults and grain-boundaries in RE-containing Mg alloys. Cottrell atmospheres were observed along dislocations. Based on atomic resolution characterizations, a binding energy of about 0.05 eV is deduced between basal dislocations and surrounding solute atmospheres. Besides providing enough independent slip systems, <c+a> dislocations can cut and react with basal stacking-faults, and react with grain-boundary dislocations, and drive migration of grain-boundaries, benefiting both strength and ductility of Mg alloys.