This research looks at the influence of annealing treatments on twins in magnesium alloy AZ31. Samples containing different twin fractions are created in order to examine the influence of the initial structure on the annealing behavior. It is seen that twins can both expand, taking over an entire grain, or shrink, leaving completely untwinned grains. The grains, containing expanding twins, are found to contain, in general, only one twin variant and limited slip deformation after twin formation. The grains containing shrinking twins, on the contrary, are seen to contain different twin variants and have undergone more slip deformation after twinning.