[en] We study cosmological models with quintessence, a form of energy that is distributed almost homogeneously in the Universe, and today makes up about three fourths of its energy density. We use observational data in order to infer upper bounds for the energy density of quintessence also during early times, and analyze the impact of this early quintessence on the interpretation of observational data. Furthermore, we investigate deviations from the law of gravity as described by the theory of General Relativity on very large scales. Finally, we consider models in which the mass of the dark matter or of neutrinos depends on the quintessence field, and present extensions of the software CMBEASY, that allow to compute the cosmological predictions of these models. (orig.)