Although less well known outside the field than Edwin Hubble, Walter Baade was arguably the most influential observational astronomer of the 20th century. Written by a fellow astronomer deeply familiar with Baade and his work, this is a biography of this major figure in American astronomy. In it, Donald Osterbrock suggests that Baade's greatest contribution to astrophysics was not, as often contended, his revision of Hubble's distance and age scales for the universe. Rather, it was his discovery of two distinct stellar populations: old and young stars. This discovery opened wide the previously marginal fields of stellar and galactic evolution - research areas that would be among the most fertile and exciting in all of astrophysics for decades to come. Baade was born, educated, and gained his early research experience in Germany. He came to the United States in 1931 as a staff member of Mount Wilson Observatory, which housed the world's largest telescope. There, he pioneered research on supernovas.With the 100-inch telescope, he studied globular clusters and the structure of the Milky Way, every step leading him closer to the population concept he discovered during the wartime year