The graduation project was conducted at the upper stage liquid propulsion department of ArianeGroup at the facilities of Airbus DS, Bremen. Based on a series of past flights of Ariane 5 launcher, the aim is to analyze the fluid motion and the pressure fluctuations in the cryogenic propulsion upper stage Liquid Hydrogen (LH₂) and Liquid Oxygen (LO_x) tanks during the ascent phase. Pressure fluctuations (drops or rises) during the ascent phase are undesirable due to the need for relief or re-pressurization of the fuel tanks. Tank relief is obtained through relief valves, while re-pressurization is done using on-board gaseous Helium; both cases increase the failure probability of the system and/or the total weight of the launcher.The objective of the project is to find out why these pressure fluctuations occur, what are the parameters that affect the pressure evolution and at what extend the liquid fuel motion (sloshing) is responsible for this behavior.According to the literature several parameters affect the pressure evolution during sloshing. These parameters are further investigated through flight data analysis. The approach also involves CFD simulations of the kinematic behavior of the liquid fuel focusing on the sloshing angle. Finally, a statistical model is built attempting to predict the pressure change inside the tanks. Higher sloshing angles match with higher pressure rise inside the LO_x tank. The magnitude of the pressure rise appears to be directly connected to the kinematic profile of the launcher as well as to the ullage volume of the tank. The maximum predicted ullage pressure is below the tank's sizing pressure limit. Regarding the LH₂ tank, no strong correlation of flight parameters to the pressure change is identified; no sufficient statistical model is built. The CFD simulation shows that relatively higher sloshing angle magnitude and duration exists near the pressure drop periods and that strong breaking waves are likely to be formed in the case of a sudden pressure drop behavior. The LH₂ tank is more prone to the formation of breaking/splashing waves. The effect of vibrations, which is not included in the CFD study, is also important for the explanation of the pressure drop magnitude.