Interfaces and interface/defect interactions increasingly dominate the mechanical response of materials as the dimensions of the grains decrease to the nanoscale. Recently, we reported unusually profuse deformation twinning in Ag-Cu layered eutectic composites with bilayer thicknesses in the submicron regime (~200 nm-400 nm) at room temperature and low strain rates. Using atomistic simulations and dislocation theory, we propose that the Ag-Cu interface facilitated deformation twinning in Cu by permitting the transmission of twinning partials from Ag to Cu. In this way, twins in Ag can provide an ample supply of twinning partials to Cu to support and sustain twin growth in Cu during deformation. Interface-driven twinning as revealed by this study suggests the exciting possibility of altering the roles of dislocation slip and twinning through the design of heterophase interface structure and properties.