For over 75 years, analytical ultracentrifugation (AUC) has proven to be a powerful method for characterizing solutions of macromolecules, and an indispensable tool for the quantitative analysis of macromolecular interactions. There are approximately 400 XLI analytical ultracentrifuges in use worldwide. These instruments are distributed almost evenly between industrial settings, primarily in pharmaceutical, biopharmaceutical, and polymer companies, and academic research facilities associated with biochemistry, biophysics, molecular biology, and material science programs. AUC provides core information for over 350 scientific publications a year (source: SciFinder 2008, using “AUC” as the search term) with usage increasing steadily (436 references in 2008).