High-field magnets - those that operate at the limits of the mechanical and/or electromagnetic properties of their structural materials - are used as research tools in a variety of scientific disciplines. The study of high magnetic fields themselves is also important in many areas such as astrophysics. Because of their importance in scientific research and the possibility of new breakthroughs, the National Science Foundation asked the National Research Council to assess the current state of and future prospects for high-field science and technology in the United States. This report presents the results of that assessment. It focuses on scientific and technological challenges and opportunities, and not on specific program activities. The report provides findings and recommendations about important research directions, the relative strength of U.S. efforts compared to other countries, and ways in which the program can operate more effectively.