We study the dislocation and twin substructures in a high manganese twinning-induced-plasticity steel (TWIP) by means of electron channeling contrast imaging. At low strain (true strain below 0.1) the dislocation substructure shows strong orientation dependence. It consists of dislocation cells and planar dislocation arrangements. This dislocation substructure is replaced by a complex dislocation/twin substructure at high strain (true strain of 0.3-0.4). The twin substructure also shows strong orientation dependence. We identify three types of dislocation/twin substructures. Two of these substructures, those which are highly favorable or unfavorable oriented for twinning, exhibit a Schmid behavior. The other twin substructure does not fulfill Schmid’s law.