The aim of the book is to provide a comprehensive and unified description of high-intensity short laser pulses and their applications at the simplest level compatible with a correct physical understanding. The idea is to provide an intuitive picture of the phenomena under consideration with simple mathematical description useful for a better understanding. The book is based on the teaching experience of the graduate course of the Politecnico di Milano "HIGH INTENSITY LASERS FOR NUCLEAR AND PHYSICAL APPLICATIONS I + II" and is particularly addressed to graduate students with a background in electromagnetism; is mostly suitable for master students in Nuclear Engineering, in Engineering Physics, and in Physics and It's recommended also to students in material sciences (or similar) and to PhD students. The text organization is due to help to follow the lessons in the classroom and to be used for self-study by students.