ch. 1. Introduction -- ch. 2. Basics of modern cosmology -- ch. 3. Active galactic nuclei -- ch. 4. Gravitational lensing -- ch. 5. Deconvolution -- ch. 6. Time delays and the Hubble constant -- ch. 7. Redshift of lensing galaxies -- ch. 8. Microlensing : a natural telescope -- ch. 9. Dynamics versus gravitational lensing -- ch. 10. General conclusions and outlook.