For steels that undergo twinning and phase transformation during deformation, the dynamic behavior can differ substantially from quasi-static behavior. As in other metals, the high strain rate response is stronger than the quasi-static response; however, in some cases the high strain rate response is weaker than the quasi-static response. In this work a variety of strain rate history tests are applied to TRIP and TWIP steel samples to reveal how the active deformation mechanisms dictate the mechanical response and depend on both the loading rate and its history. Complimentary microscopy is performed to quantify the relative activity of slip, twinning, and phase transformation in samples. Modifications to existing constitutive models are proposed to provide a rationale for the perceived dynamic softening in some TRIP steels.