This monograph presents a comprehensive overview of High Power Lasers – Directed Energy Weapons that combines the best of optics and electronics to offer unprecedented capabilities. High Power Laser (HPL) has emerged as one of the very promising technologies that will have major impact on defence and security in the future. The DEW with its unique features is clearly the new generation weapon that will transform the concepts of defence and deterrence. This monograph presents a comprehensive overview of this complex subject.