  The holographic correspondence creates an interface between classical
gravitational physics and the dynamics of strongly interacting quantum field
theories. This chapter will relate the physics of charged, asymptotically
Anti-de Sitter spacetimes to the phenomenology of low temperature critical
phases of condensed matter. Common essential features will characterise both
the gravitational and field theoretic systems. Firstly, an emergent scaling
symmetry at the lowest energy scales appears as an emergent isometry in the
interior, `near horizon' regime of the spacetime. Secondly, the field theoretic
distinction between fractionalized and mesonic phases appears as the presence
or absence of a charge-carrying horizon in the spacetime. A perspective
grounded in these two characteristics allows a unified presentation of
`holographic superconductors', `electron stars' and `charged dilatonic
spacetimes'.
