The decay of concentration fluctuations in polymer solutions is investigated. Starting from a microscopic basis, a theoretical framework is developed to describe so-called memory effects leading to additional slow modes. The theory is formulated for a multicomponent system. These general results are applied to a homopolymer-solvent system. With respect to the interpretation of dynamic light scattering the behavior at small scattering vectors q is relevant. In this case, an approximately diffusive decay mode and slower, approximately q-independent modes related to viscoelastic properties are predicted at higher polymer concentrations. The relation to typical experimental results and to the theory recently presented by Wang is discussed.