  The aim of this paper is to explain carefully the arguments behind the
assertion that the correct quantum theory of gravity must be background
independent. We begin by recounting how the debate over whether quantum gravity
must be background independent is a continuation of a long-standing argument in
the history of physics and philosophy over whether space and time are
relational or absolute. This leads to a careful statement of what physicists
mean when we speak of background independence. Given this we can characterize
the precise sense in which general relativity is a background independent
theory. The leading background independent approaches to quantum gravity are
then discussed, including causal set models, loop quantum gravity and dynamical
triangulations and their main achievements are summarized along with the
problems that remain open. Some first attempts to cast string/M theory into a
background independent formulation are also mentioned.
  The relational/absolute debate has implications also for other issues such as
unification and how the parameters of the standard models of physics and
cosmology are to be explained. The recent issues concerning the string theory
landscape are reviewed and it is argued that they can only be resolved within
the context of a background independent formulation. Finally, we review some
recent proposals to make quantum theory more relational.
