The gamma surfaces in the pyramidal I {1 −1 0 1} and II {1 1−2 2} planes for hexagonal close packed Mg have been calculated using two embedded-atom-method potentials and by ab initio methods, and reasonable agreement is obtained for key stacking fault energies. Screw and edge 〈c + a〉 dislocation core structures and Peierls stresses at 0 K and finite temperature have been examined using the embedded-atom-method potentials. Screw 〈c + a〉 dislocations glide in the {1 −1 0 1} pyramidal plane I, and in the prism plane for larger stresses, but not in the {1 1 −2 2} plane as observed in experiments. However, the preference for pyramidal I glide correlates well with the gamma surfaces. New low energy edge 〈c + a〉 dislocation cores were found in addition to the sessile Type I and Type III cores observed in previous simulations while the Type II core was not observed. The lowest energy core is a glissile core that lies in the {1 1 −2 2} plane and has a 3 nm long {1 1 −2 1} twin embryo, rather than the sessile Type III core found in previous simulations. As the temperature increases from 0 to 300 K, the Peierls stresses in compression/tension drop from −80 MPa/+140 MPa and −140 MPa/+220 MPa for the most glissile screw and edge dislocations to −5/+2.5 MPa and −27/+5 MPa, and dislocation glide changes from kink motion to face-centered-cubic-like motion. At 300 K and under an applied stress, almost all the edge cores found at low temperature transform into a glissile core denoted IT, which glides at low stresses. Thus, at 300 K both screw and edge 〈c + a〉 dislocations were found to glide at stresses smaller than the not, vert, similar40 MPa measured experimentally.