Direct Methanol Fuel Cells (DMFC) are analysed experimentally and theoretically with respect to their steady-state and dynamic operating behaviour. The current status (2003) of the DMFC is presented with special focus on basic principle, functional materials of which the DMFC consists and modeling and simulation approaches. An own laboratory scale DMFC design is presented, as well as a fully automated miniplant used for its operation under various steady-state and dynamic operating conditions. The miniplant allows the determination of methanol and water crossover fluxes from anode to cathode. The DMFCs are fed with liquid methanol water solutions and air. Influences of methanol feed concentration, temperature, pressure and electric cell current are analysed using a rigorous one dimensional process model of the DMFC. In this model the generalised Maxwell-Stefan equations are used for describing mass transport in porous structures. A special focus lies on the realistic description of the polymer electrolyte membrane (PEM), where in the model even swelling and phase equilibria at the interfaces are accounted for. The results show that methanol and water crossover through the membrane are governed by diffusion rather than electro-osmosis, and that the model yields good approximations to experimental results.