My task is to discuss the basic principles of Quantum Mechanics which are crucial for the electronic structure theory. The following topics will be covered: the correspondence principle which connects Classical Mechanics and Quantum Mechanics; the uncertainty principle and related questions; the superposition principle. We shall discuss the Hilbert space of wavefunctions, and the operators associated with the observables. We shall illustrate the theory by considering the properties of angular momentum (orbital and spin). The theory of hydrogen atom will constitute the important part of the lecture.