In keeping with goal and style of the Handbook in Modern Biophysics series, the proposed book will maintain a chapter structure that contains two parts: concepts and biological application. The book also integrates all the chapters into a smooth, continuous discourse. The first and second chapters establish the mathematical methods and theoretical framework underpinning the different topics in the rest if the book. Other chapters will use the theoretical framework as a basis to discuss optical and NMR approaches. Each chapter will contain innovative didactic elements that facilitate teaching, self-study, and research preparation (key points, summary, exercise, references).