A relativistic theory of gravity has been proposed by Bekenstein, aimed at solving the missing mass problem. In this talk I review the key features of the theory. I then discuss issues that this theory and all modifications to gravity have to face such as ghosts, stability and positivity of energy. I will then present what observations have to say about the current versions of the theory. I will finish with an outlook on how future solar system and cosmological observations can constrain modifications to gravity.