 In non-generally relativistic theories the spacetime metric is frozen to a single value assignment for all times and all solutions: it is model independent. Of course, in general relativity the metric is what one solves for: the metric is a dynamical variable, which implies that the geometry of spacetime is dynamical. This intuitive notion is bundled into the concept of background freedom, or background independence. In general, background independence is understood to be the freedom of a theory from background structures, where the latter amount to some kind of absolute, non-dynamical objects in a theory. The extent to which their respective theories incorporate background structures has recently proven to be a divisive subject amongst string theorists and loop quantum gravity theorists and others. It is often claimed that the central principle that distinguishes general relativity from other theories is its (manifest) background independence. But background independence is a slippery notion meaning different things to different people. We face a series of questions when considering background independence: What, exactly, is it (beyond the simple intuitive notion)? Why is it considered to be such an important principle? What theories incorporate it? To what extent do they incorporate it?