D ISTRIBUTIONS are essential tools for the description of most SV experi-ments due to the immense information content and exquisite sensitivity of SV data for sample heterogeneity. For example, it is quite rare that a biological sample, such as a protein preparation, is truly monodisperse, free of breakdown products, and does not contain at least low levels of aggregates detectable by SV. Further, many of the more interesting experimental questions are directed at the formation of new species due to attractive interactions between different components in a mixture. In other disciplines, such as polymer chemistry, the need for models that capture the polydisperse nature of the particle ensemble under study is even more obvious, due to the lack of a highly specific synthesis machinery producing monodisperse macromolecules.