This paper presents what is our actual knowledge in the development of the on-line control of the plasma spray process. First the influence of the different parameters on coating properties is recalled. The dependence of particle parameters at impact on macroscopic parameters such as the gas composition, its enthalpy, the injection conditions… is discussed together with the possible actions to compensate for the voltage drop due to electrodes erosion. The transient aspect of the plasma jet due to the arc root fluctuations and their consequence on particle parameters is also presented. Then the different sensors able to work in booths harsh environment are described. The way they are used, informations they provide, the possibility they give to monitor the spray parameters for a good working area is discussed. At last what would really be necessary for a real on-line control is presented and it is concluded that we are still far to achieve this objective.