Ever increasing requirements for vehicle performance, fuel economy and emissions have been driving the development and adoption of various types of hybrid powertrains. There are many different configurations of hybrid powertrains, which may include such components as engine, generator and inverter, battery pack, ultracapacitor, traction motor and inverter, transmission, and various control units. A hardware-in-the loop (HiL) testing solution that is flexible enough to accommodate different types of hybrid powertrain configurations and run a range of test scenarios is needed to support on-going development activities in this field.This paper describes the design and implementation of such a HiL testing system. The system is centered on a high performance, real-time controller that runs powertrain, driveline, vehicle, and driver models. Multiple dynamometers are included in the platform which can be used to load the powertrain and emulate the dynamic effects of vehicle and driveline components not physically present in the setup. A drive unit is integrated in the system. Based on different testing scenarios, this unit could be used to simulate the battery pack or other sources of energy, manipulate the power flow in the hybrid powertrain and emulate the effects of engine/generator, vehicle accessory loads, or different efficiency profiles of traction motors. By combining necessary simulations and controls, the system can be structured to test various hybrid powertrain configurations, for as few or as many of the powertrain components as needed, and to implement different testing scenarios. A typical application shows that the system could be a powerful tool for various hybrid powertrain development needs.