The introduction to the basic theories of Physics of our century - Relativity and Quantum Mechanics - is the subject of this widely known and used book "Modern Physics". The level of mathematical treatment of the material (clearly higher than what is customary in General Physics books), the clarity of the presentation and the abundance of examples are some of the advantages of the book, which explain its success both among Physics students and those of Technical Schools. The book is also considered particularly successful in the application of the fundamental laws of Quantum Mechanics and Relativity for understanding the basic characteristics of matter at all levels of its organization: elementary bodies, nuclei, atoms, molecules, solids. The new technologies that have emerged from this understanding - lasers, semiconductor devices, superconductors, etc. - constitute the third main axis on which the book is developed and completed. The edition is two-color with approximately three hundred and fifty graphic representations, tables and images.