Hybrid transmission system

In a hybrid drive system (A) for a work vehicle (F) with a mechanical drive train between an internal combustion engine (5) and a drive axle (2) as well as a parallel hydraulic hybrid module (H), which has a hydraulic unit (11) operable as a motor or pump, connected via a clutch (10) to the mechanical drive train (4), and at least one pressure accumulator (HS1, HS2), a hydraulic work module (M) is provided for the work vehicle and is connected or connectable to the hybrid module (H).