One of the major accomplishments of laser technology that took place dur­ ing the last 15 years is the possibility of generating coherent radiation that 18 can be focused so that its intensity reaches the magnitude of 10 W/ern". Even higher intensities result from nonlinear self-focusing of such radiation in matter. The unique character of these magnitudes is illustrated by the fact that these intensities are substantially higher than those occurring inside the Sun. A range of previously unexplored physical mechanisms come into playas laser pulses interact with matter under the conditions of an extreme concen­ tration of laser energy. In particular, free electrons of plasma formed by rapid nonlinear ionization of gases and solid targets during the pulse rise time are driven by a laser radiation electric field at velocities comparable to the speed of light, and the corresponding relativistic increase in their masses entails a modification of the plasma's optical properties. Laser radiation intensities at which the above effect occurs are called relativistic. This book is intended to provide an introduction to the field of laser physics at relativistic intensities. Extensive theoretical and experimental studies have been performed in this area in the last decade. At present, laser physics at relativistic intensities can be considered a new and rapidly evolving area of modern physics. Impor­ tant basic new phenomena and concepts of applications are associated with it, among them relativistic and charge-displacement self-channeling, wake­ field particle acceleration, generation of free electron harmonics and X-rays, and fast ignition.