Contents. 1. Operators theory in finite-dimensional vector spaces. 2. Perturbation theory in a finite-dimensional space. 3. Introduction to the theory of operators in Banach spaces. 4. Stability theorems. 5. Operators in Hilbert spaces. 6. Sesquilinears forms in Hilbert spaces and associated operators. 7. Analytic perturbation theory. 8. Asymptotic perburbation theory. 9. Perturbation theory for semigroups of operators. 10. Perturbation of continuous spectra and unitary equivalence.