As we have progressed through the story set out in this book, we have met and begun to understand many of the objects that astronomers regularly photograph: planets, stars, galaxies, supernovae. Astronomical photographs show, in fact, the astonishing variety of objects that make up our Universe. But, to my eye, the most spectacular and entertaining astronomical photographs are fashioned by the objects we will study in this chapter: gravitational lenses. Let's start this chapter with two, shown in Figure 23.1 on the following page. Gravitational lenses are a spectacular illustration of the working of general relativity in the Universe. And besides entertaining us with pictures of eerie beauty, they have become an important tool of astronomy, a way of probing the distribution of mass (and in particular the dark matter) in galaxies and clusters of galaxies.