In situ transmission electron microscopy observations are reported of the dynamic process of twin boundary migration in Cu with nanoscale twins. The experiment provides the first direct evidence of twin boundary migration via Shockley partial dislocation emission from the twin boundary/grain boundary intersections, and reveals that such migration is the dominant deformation mechanism in the initial stage of plastic straining. The behaviour is discussed in comparison with molecular dynamics simulations and in terms of the unique characteristics of the sample microstructure.