This festschrift volume is dedicated to William Liller, a noted observational astronomer who is now retired from a Harvard University named professorship. Nineteen of Liller’s colleagues and former students (as well as Liller himself) give insightful reviews of the current state of a broad range of frontier astrophysical areas from the solar system to the limits of the observable universe. The reviews are of special interest to professional astronomers and students of astronomy, and are also accessible to the interested nonspecialist reader. The articles are well suited for providing graduate students in astronomy with an introduction to these topics. The authors describe their personal involvement in important research advances and convey a sense of the excitement of scientific discovery. The reader is given an overview of the advanced techniques used by today’s observational astronomers for probing the mysteries of the cosmos, and of the theoretical interpretation of the observational findings.