This book contains papers of the outstanding and versatile mathematician, Witold Hurewicz. Preceding the collection are introductory articles describing Hurewicz's contributions to Borel sets, dimension theory, and algebraic topology. Hurewicz first studied set theory and dimension, and his papers on this topic are especially clear and precise, making them accessible to beginning mathematicians. His work in algebraic topology is marked by five fundamental papers which provide an introduction to homotopy groups and the Hurewicz Theorem concerning the relation between homotopy and singular homology. These papers are included here in their original form along with English translations. Each paper in the collection is followed by a review from one of the major reviewing journals. These reviews were written by eminent mathematicians and serve as excellent abstracts for the papers.