Selected by 50 notable astronomers from the major sub-fields of the discipline, the articles assembled in this special AAS Centennial collection are accompanied by commentary that provides the scientific-historical context essential to comprehending each article's original impact. Many commentators were contemporaries of the original authors and provide first-person accounts of papers published in the journals—and the earliest reactions they evoked. Arranged in chronological order of publication, these classic papers include works by Subrahmanyan Chandrasekhar, George E. Hale, Fred Hoyle, Edwin Hubble, A.A. Michelson, Henry Norris Russell, Arthur Achuster, Harlow Shapley, and others. Together the articles and commentaries provide a historical window into twentieth-century astronomy and how the results were achieved.