Holography can be used to study the physics of strongly coupled quantum
field theories at finite density, which may be at work in certain real
materials. However, the conservation of momentum in these field theories
means that they have unrealistic transport properties, such as an infinite
electrical conductivity. I will give a summary of recent progress in
constructing gravitational solutions which are dual to field theories
which dissipate momentum. I will explain how these gravitational solutions
are related to theories of massive gravity, and what the transport
properties of the dual field theories are. Finally, I will describe a
simple mechanism, which is natural from the holographic point of view, by
which strongly interacting metals can acquire a linear in temperature
resistivity, as is observed in the strange metal phase of the cuprates.