The distribution of galaxies, mapped by redshift surveys, is a powerful observable which is used to probe some of the fundamental properties of our Universe. In order to exploit this quantity correctly it is essential to understand what we are really measuring when we look at maps of galaxies. Since our Universe is not perfectly homogeneous and isotropic, we only have access to a distorted picture of our sky. In this talk, I will discuss the various relativistic effects that deform our observations. I will show that these effects are sensitive to specific features of our Universe and that we can consequently use them to test some of the fundamental properties of gravity.