Models are intended to reduce the complicated structure of natural systems in order to make more transparent those processes which are relevant to a given question. Due to the concept of the "limiting nutrient" the process of lake eutrophication can be described by fairly simple phosphorus models. Starting from the model of mass balance two ways serve to render the models more realistic, (1) the introduction of chemical-biological transformations (photosynthesis, mineralisation, ecological interaction such as between phytoplankton and zooplankton); (2) the description of mixing processes within the system. In both cases the availability of field data is eventually limiting the complexity of the model. It is important to adjust the complexity of the chosen model to the question under discussion and to the possibility of model verification by field data.