Martensite formation in ferrous alloys is inextricably connected with local deformation. In order to assess the role of this plastic work in the transformation, the dynamic responses of Fe−Ni−C austenites to imposed changes in deformation variables are investigated. For both stable and metastable compositions direct measurements are made of the separate and combined influences of prestrain and applied stress, and of strain-rate and dynamic strain aging. In addition, new martensite-start temperatures are defined and examined conceptually. A differential form of an energy balance that describes our viewpoint of austenite transformation at stress is then used to interpret the results. Some consequences concerning the importance of plastic work, its structure sensitivity, and the pervasive influence of local stress states are deduced and discussed.