Gravitational lensing is a consequence of light deflection in gravitational field. Starlight deflection by the Sun, measured as 1′′.7 at the solar limb was the first evidence in favor of General Relativity. This result is often presented in contrast with so called Newtonian prediction dated back to Soldner, but in fact it announces much more by its very existence. Namely that the nature of gravity lies in geometry of spacetime shaped by distribution of mass and energy, according to the Einstein Equations. The so called Newtonian prediction is, in some sense, internally inconsistent since photons are massless so why should they feel gravity of the Sun while passing by. In relativistic context things are clear: photon’s path as a geodesic in a curved spacetime is no longer a straight line. General Relativity provided also a framework in which cosmology has been promoted from just philosophical speculations to a solid branch of physical science. Namely, we can define the goal of cosmology as studying structure of the space-time at largest scales, indeed even its global structure. First gravitational lenses were discovered in cosmological context: as multiple images of a quasar produced by foreground galaxy or as arcs around galaxy clusters representing parts of Einstein rings. Hence it is no surprise that gravitational lensing has great potential in constraining cosmological model and/or alternative theories of gravity. This chapter starts with an introduction to the present cosmological model and its two unsolved problems of dark matter and dark energy. Especially the accelerating expansion of the Universe is a great challenge for both physics and cosmology. In light of lacking the convincing theoretical explanation, an effective description of this phenomenon in terms of a cosmic equation of state turns out useful. The strength of modern cosmology lies in consistency across independent, often unrelated pieces of evidence. Therefore, every alternative method of restricting the cosmic equation of state is important. Strongly gravitationally lensed quasar-galaxy systems create such a new opportunity by combining stellar kinematics with lensing geometry. The utility of strongly lensed systems (galaxy and cluster lenses) for cosmology will be discussed. Then an emphasis will be put on using strong gravitational lenses as probes of cosmic equation of state which is becoming attractive in light of ongoing surveys like SLACS based on different protocols than older searches focused on potential sources. In this 6