The competition between dislocation emission and cleavage at a crack tip plays an important role in governing the intrinsic fracture behavior of crystalline materials. This competition is not well understood in magnesium, which has many different combinations of cleavage planes and dislocation slip systems. Here, using both anisotropic linear elastic fracture mechanics theory and atomistic simulations, the emission/cleavage competition in magnesium is evaluated for a comprehensive set of crack orientations and crack tip geometries under mode I crack tip stress intensity loading at T= 0 K. Both theory and simulation show that cleavage is favored in most crack orientations, including basal plane cracks, tensile twin and basal-prismatic plane interface cracks. Initial slight crack tip blunting does not significantly change the behavior. These results suggest that magnesium has extremely low intrinsic fracture toughness, consistent with observations of many different cleavage-like planes in low-temperature fracture experiments. Based on T = 0 K properties, obtaining a more-ductile material via crack tip dislocation emission on the pyramidal and basal slip systems would require substantial and moderate reductions of the unstable stacking fault energy (similar to 50% and similar to 20%, respectively) to be achieved at finite temperatures and/or via alloying. (C) 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.