The morphology and crystallography of lath martensite in low and medium carbon steels have been studied by transmission electron microscopy and diffraction. The steels have microduplex structures of dislocated lath martensite (a M direction. The overall microstructure and orientations result from minimization of the total strain and shape de formation. Considerable accommodation occurs by de formation of laths (sometimes twinned) and austenite (sometimes tripped to twin martensite). In the meantime, microchemical analyses have shown considerable carbon segregation to the martensite-austenite interface.