William Thurston's work has had a profound influence on mathematics. He connected whole mathematical subjects in entirely new ways and changed the way mathematicians think about geometry, topology, foliations, group theory, dynamical systems, and the way these areas interact. His emphasis on understanding and imagination in mathematical learning and thinking are integral elements of his distinctive legacy. This four-part collection brings together in one place Thurston's major writings, many of which are appearing in publication for the first time. Volumes I-III contain commentaries by the Editors. Volume IV includes a preface by Steven P. Kerckhoff. Volume I contains William Thurston's papers on foliations, mapping classes groups, and differential geometry.