An SEM in situ uniaxial tensile testing setup allowing HR-EBSD acquisition during deformation was used to study the extension twinning mechanism in magnesium(Mg) at the micron scale. Structures were fabricated with two different crystal orientations, respectively perfectly aligned with, and at 5 degrees to, the [0001] axis. Limited {10 (1) over bar2} twin formation was identified in the former case, while twinning was found to largely accommodate the plastic deformation in the latter case. These two different mechanisms are explained by the activation of basal slip when loading at 5 degrees to the c-axis, which triggers {10 (1) over bar2} twin nucleation and favors twin growth and propagation. The other orientation shows the activation of pyramidal slip together with only limited {10 (1) over bar2} twin growth. The critical resolved shear stress for {10 (1) over bar2} twinning has been determined to be ten times higher than in bulk material. 3D HR-EBSD mapping enabled reconstruction of the three dimensional twin structure after deformation. From this, the interaction between the dislocations located ahead of the incoming twin and a pre-existing twin boundary was investigated, where the GND distribution and the local shear stress were determined. The results show plastic accommodation up to similar to 11% of strain, revealing higher ductility than usually reported for bulk materials. (C) 2020 The Authors. Published by Elsevier Ltd.