Clutch shift control is critical for efficient and high-performance transmission designs, including automatic, dual clutch, and hybrid transmissions. To ensure a smooth clutch to clutch shift, appropriate controls for two consecutive processes are critical. One is the precise coordination between the on-coming and off-going clutches, which requires the on-coming clutch to be filled and ready for engagement at the predetermined time (clutch fill). The other is the proper torque control during clutch engagement. In this paper, we will investigate the closed-loop wet clutch control enabled by a pressure sensor in the clutch chamber. The main challenges of the pressure-based wet clutch control lie in the complex nonlinear dynamics due to the interactions between the fluid and the mechanical systems, the ON/OFF behavior of the clutch assembly, the time-varying clutch loading condition, the required short time duration for a precise and robust clutch shift, and the lack of the displacement information. To enable precise and robust pressure-based control, this paper focuses on the following three aspects. First, a clutch dynamic model is constructed and validated, which precisely captures the system dynamics in a wide pressure range. Second, a sliding-mode controller is designed to achieve robust pressure control while avoiding the chattering effect. Finally, an observer is constructed to estimate the clutch piston motion, which is not only a necessary term in the nonlinear controller design but also a diagnosis tool for the clutch fill process. To validate the proposed methods, a transmission clutch fixture has been designed and built in the laboratory. The experimental results demonstrate the effectiveness and robustness of the proposed controller and observer. © 2012 IEEE.