The Festschrift in celebration of John William Helton's pioneering work, Mathematical Methods in Systems, Optimization, and Control comprises a careful selection of articles on systems and control theory, operator theory, and optimization. The book follows Helton’s original and long lasting contributions which have shaped control theory and operator theory during the last decades, with specific emphasis on topics aimed at direct applications to current research areas of high impact and interest. The book, by virtue of its diverse contents, strong interdisciplinary contents, and high expertise of its contributors, is one of a kind among all recent publications in the area of mathematical control theory. Mathematicians of all denominations (algebraists, geometers, analysts, numerical and computer scientists) as well as engineers interested in control and optimization will find invaluable references and sources of inspiration in the book.