This book is specially concerned with fundamental theoretical and experimental aspects of relativistic beam physics, recoil, and cooling phenomena in atomic and ion beams and traps, with emphasis on coherence and collective effects. The central theme is the physics of atomic laser and free electron laser, and the development of a bridge between them through the mechanism of the so-called recoil induced gain mechanism. The links between relativistic beam physics and atomic laser physics are explored. This book will be directed to an audience of non specialists or to specialists in only one of the fields mentioned above. It addresses the following topics: - Fundamentals of laser cooling and trapping of atoms: theory and experiments; - Quantum optics and atomic coherence effects; - Laser cooling of trapped ions; from single ion to ion crystal; - Spatio-temporal instabilities in optical systems; - Coherence in atom optics; atomic diffraction and interferometry; - Optical lattices; nonlinear effects in laser-cooled atoms; - Coherent population trapping; - Two-level gain and collective recoil-induced effects; - Fundamental physics of relativistic particles beams; - High-gain free electron laser: theory, experiments and projects; - Cooling of ion beams in a storage ring; - Experiments on dense laser-cooled stored ion beams.