With the Hubble Space Telepscope's next servicing mission still uncertain, identifying the most crucial science to be performed by this superb telescope has become of paramount importance. With this goal in mind, this book presents a review of some of the most important open questions in astronomy today. World experts examine topics ranging from extrasolar planets and star formation to supermassive black holes and the reionization of the universe. Special emphasis is placed on what astronomical observations should be carried out during the next few years to enable breakthroughs in our understanding of a complex and dynamic universe. In particular, the reviewers attempt to identify those topics to which the Hubble Space Telescope can uniquely contribute. The special emphasis on future research makes this book an essential resource for both professional researchers and graduate students in astronomy and astrophysics.