Many properties of condensed phase systems are significantly influenced by interactions with the environment, which should be, hence, taken into account in a theoretical treatment. In this context, the popular Caldeira-Leggett model, which gives a compact characterisation of the environment in terms of a spectral density, constitutes a unified framework for treating the reduced system dynamics via different methods. This thesis addresses fundamental questions about the computation of spectral densities and the applicability of the model to realistic condensed phase environments.<eng>
