Hellmuth Kneser (1898-1973) is the second of three mathematicians from consecutive generations of the Kneser family, all of them with groundbreaking mathematical contributions on a wide range of topics. It was only in recent times that mathematicians recognized how Hellmuth Kneser's work influenced the course of topology and the theory of several complex variables in the 20th century. Indeed he was a mathematician of extraordinarily broad vision and insight and thus contributed to many mathematical fields of pure and applied mathematics including foundations, differential equations, operations research, and mathematics education. With the exception of two papers written in French, all of his articles were written in German. Not all of them are readily available through the usual sources. For this book, presenting the entire collection of Kneser's papers published in journals, experts in various areas have written English commentaries on aspects of Hellmuth Kneser's work, summarizing what he accomplished, describing the context of his work, and giving outlooks on its aftereffects.