Aberration corrected STEM was used to investigate stacking faults in TWIP steel. Extrinsic stacking faults were observed in 〈100〉 grains parallel to the tensile axis. ESFs serve as nuclei for deformation twins and also contribute to twin growth. Twinning in 〈100〉 grains is not necessarily a deviation from Schmid's law. Twinning via intrinsic stacking faults was also observed in 〈100〉 grains.