The austenite/martensite (fcc/bcc) interface is prevalent across many new classes of high-strength steels, and yet both its fundamental structure and its mechanism of motion remain uncertain in spite of decades of research. Here, atomistic simulations are used to create an fcc-bcc iron interface having a structure and motion that match the major experimental observations on dislocated lath martensite. The simulated interface reveals a defect structure and a mechanism of glissile and athermal propagation that differ in important respects from longstanding assumptions. The atomisticallyobserved interface defects provide a basis for a parameter-free predictive crystallographic double shear theory of lath martensite. Predictions of the theory match simulations well and yield very good agreement with experiments on Fe-Ni-Mn and Fe-C. The theory shows that the fcc/bcc lattice parameter ratio is the dominant factor for controlling the "shape" deformation the overall, in-situ strain associated with the martensitic transformation which is related to macroscopic toughening, and quantitatively rationalizes many experimental observations. This new understanding about the nature of this special interface provides fundamental insights needed for guiding design of emerging high-strength steels. (C) 2017 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.