  Frictional interfaces are abundant in natural and engineering systems, and
predicting their behavior still poses challenges of prime scientific and
technological importance. At the heart of these challenges lies the inherent
coupling between the interfacial constitutive relation -- the macroscopic
friction law -- and the bulk elasticity of the bodies that form the frictional
interface. In this feature paper, we discuss the generic properties of the
macroscopic friction law and the many ways in which its coupling to bulk
elasticity gives rise to rich spatiotemporal frictional dynamics. We first
present the widely used rate-and-state friction constitutive framework, discuss
its power and limitations, and propose extensions that are supported by
experimental data. We then discuss how bulk elasticity couples different parts
of the interface, and how the range and nature of this interaction are affected
by the system's geometry. Finally, in light of the coupling between interfacial
and bulk physics, we discuss basic phenomena in spatially-extended frictional
systems, including the stability of homogeneous sliding, the onset of sliding
motion and a wide variety of propagating frictional modes (e.g. rupture fronts,
healing fronts and slip pulses). Overall, the results presented and discussed
in this feature paper highlight the inseparable roles played by interfacial and
bulk physics in spatially-extended frictional systems.
