This text covers the crest lines of the physics of the 21st century. It is an overview, written by some of the world's leading physicists, of the late-1990s status of understanding in a number of important and interesting fields in modern physics. These include topics from astrophysics, particle physics, cosmology, biophysics, high energy physics and theoretical physics. The contributors include Steven Chu, winner of the 1997 Nobel prize. This collection consists of individual contributions by physicists writing about what they see as the important questions and areas in their specialized fields of research. Each contribution is relatively self-contained (but where appropriate, contributors have been encouraged to discuss their chapters with contributors of related chapters), and highlights particular problems in the field that are of significance.