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° to, the 0001 axis. Limited {101¯2} 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° to the c-axis, which triggers {101¯2} twin nucleation and favors twin growth and propagation. The other orientation shows the activation of pyramidal slip together with only limited {101¯2} twin growth. The critical resolved shear stress for {101¯2} 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 ~11% of strain, revealing higher ductility than usually reported for bulk materials. Display omitted •In situ HR-EBSD was used to map the sequence of events that occur during micro-tensile loading of single crystal Mg.•Extension twins occur at the micron-scale when loaded off the c-axis, while limited twins form when loaded along the c-axis.•The critical resolved shear stress for {101¯2} twinning has been determined to be ten times higher than in bulk material.•Basal slip triggers {101¯2} twin nucleation and strongly favors twin growth and propagation.•The 3D structure of the twins was obtained by FIB tomography and HR-EBSD allows associated stress and dislocations analyses.