This volume includes 20 contributions of the 12th meeting on Analytical Ultracentrifugation from March 1-2, 2001 in Duisburg, Germany. Various fields of ultracentrifugation are covered concerning research problems in biochemistry, biophysical chemistry and macromolecular chemistry as well as interacting systems. New investigations concerning the sedimentation theory are presented. The phase transition of gels is dealt with, as is the sedimentation-diffusion equilibrium of gels. One section contains the hydrodynamics of biopolymers.