Quantum Mechanics is a fundamental scientific discipline. At the same time, it is viewed as being very difficult. This book attempts to present the Theory of Quanta as a scientific discipline which has emerged naturally from experiment, making use of general concepts of the Classical Mechanics and enlarging their nature and applicability. According to the historical development, the first natural way of introducing the quantum-mechanical concepts is the matricial theory, followed by the very useful approach of the undulatory theory. The core of the Theory of Quanta is the quasi-classical theory, described at large in the book. The book does not circumvent the so-called philosophical problems of the Quantum Mechanics.