A new real-time substructure method for testing systems under dynamic loading is described. The method separates a complex system into a physical, possibly nonlinear, subsystem (to be tested experimentally at large scale) and a surrounding linear system, modeled numerically. The two subsystems interact in real time allowing realistic time-dependent nonlinear behavior to take place. This behavior may be impossible to model computationally, and the method overcomes problems associated with scaling and time-dependent effects associated with current shaking table and pseudodynamic test methods. Dynamic forces are applied to the numerical model, and the resulting displacements at the interface between the two subsystems are applied to the physical system using servohydraulic actuators. The restoring forces are then measured and fed back to the numerical model, so that the response for the next time step can be calculated. The technique can be applied to a wide range of complex, linear/nonlinear systems such as structures with localized plastic deformation, soil-structure interaction, or vehicle-suspension interaction. The method is demonstrated for both single and multi-degree-of-freedom systems, using a single actuator to apply displacements to a physical specimen. Comparisons between experimental and theoretical results show excellent agreement.