The demand for air travel is increasing as more people gain access to commercial aviation. As the current generation of aircraft makes use of fossil fuel combustion, this growth results in an increase in the global emissions. This is at odds with the worldwide efforts of reducing the adverse effects of climate change. Therefore, advanced propulsion systems must be developed to limit the emissions caused by the commercial aerospace industry. Using hydrogen fuel cells for propulsion is a promising technology to potentially get the sector to zero emissions. It is of interest to explore the capabilities and feasibility of aircraft with a hydrogen fuel cell powertrain. To determine the feasibility for a wide range of aircraft, a general design methodology is required.Current research efforts focus on component level performance, however system level design research, while present, didn't introduce a general methodology. The most pressing challenge was found to be related to the system level design of a CS-23 category hydrogen fuel cell aircraft. The CS-23 category aircraft class has been identified as the most suitable focus for research efforts, due to the lower technical and certification requirements placed on the components to reach a feasible design. A general methodology for the design of CS-23 category aircraft was therefore found to be a useful contribution to the state of the art. This additionally provides a deeper understanding into the most important parameters of the power and propulsion systems design.In this report, a general methodology for the conceptual design of hydrogen fuel cell powered CS-23 category aircraft is presented. The methodology makes use of a modified class 1 weight estimation for initial sizing according to customer requirements and component technology levels. The generated aircraft is refined using further aerodynamic analysis, which results in a feasible aircraft concept, as well as component level specifications for important aircraft components.The methodology is implemented in a software tool, HAPPIE (Hydrogen Aircraft Power \& Propulsion Initial Estimator), which allows for rapid sizing of different hydrogen fuel cell concepts. This makes the methodology accessible, and additionally provides feedback on the effects of individual design choices and technology levels on system level performance. SUAVE is used to perform the refined aerodynamic analysis.The methodology is validated using existing conventionally powered aircraft, by comparing the sizing results from the methodology with publicly available data. The methodology's ability to analyse conventional as well as hydrogen fuel cell powertrains, furthermore allows for performance comparison between current and future technologies.The results of the sizing methodology demonstrate the viability of hydrogen fuel cell aircraft in the CS-23 category. A conceptual design is generated, which serves as a baseline for the sensitivity analyses. It is found that hydrogen fuel cell aircraft are generally heavier than conventional aircraft, using current technology levels. Liquid hydrogen is identified as the best hydrogen storage method. Compressed hydrogen storage is also possible, however this results in a heavier aircraft with limited range. The current methodology does not predict thermal behaviour to have a significant effect on the mass of the aircraft. A component sensitivity analysis determined that the fuel cell efficiency, fuel cell specific power and hydrogen storage efficiency are the most important parameters.The ideal cruising altitude for fuel cell aircraft is at an intermediate altitude, due to the fact that the fuel cell powertrain performance decreases at increasing altitude, which balances with the lower drag in lower density air.The research demonstrates that hydrogen powered CS-23 aircraft are viable for current technology levels, and are a suitable way in reducing carbon emissions in this category.