Gravitational lensing has gained increasing importance in the last decades, although it was considered as a mere theoretical curiosity until the late 1970s. Several recent breakthroughs in cosmology can be attributed to gravitational lensing. Indeed, gravitational lensing and in particular gravitationally lensed quasars offer numerous applications to address fundamental subjects of modern cosmology such as the Hubble constantH0, dark matter or dark energy to name a few.