A hybrid drive system includes a transmission having first and second parallel input shafts. The first and second parallel input shafts are selectively coupled together via a plurality of gearwheels operable to vary the rotational speed of the second parallel shaft relative to the first parallel shaft. The hybrid drive system includes at least one irreversible drive machine coupled to the transmission and operable to provide torque to at least one of the first or second parallel input shafts, and at least one reversible drive machine coupled to the transmission and operable to provide torque to at least one of the first or second parallel input shafts.