  We review the current state of efforts to use resonant soft x-ray scattering
(RSXS), which is an elastic, momentum-resolved, valence band probe of strongly
correlated electron systems, to study stripe-like phenomena in copper-oxide
superconductors and related materials. We review the historical progress
including RSXS studies of Wigner crystallization in spin ladder materials,
stripe order in 214-phase nickelates, 214-phase cuprates, and other systems.
  One of the major outstanding issues in RSXS concerns its relationship to more
established valence band probes, namely angle-resolved photoemission (ARPES)
and scanning tunneling microscopy (STM). These techniques are widely understood
as measuring a one-electron spectral function, yet a relationship between RSXS
and a spectral function has so far been unclear. Using physical arguments that
apply at the oxygen $K$ edge, we show that RSXS measures the square modulus of
an advanced version of the Green's function measured with STM. This indicates
that, despite being a momentum space probe, RSXS is more closely related to STM
than to ARPES techniques.
  Finally, we close with some discussion of the most promising future
directions for RSXS. We will argue that the most promising area lies in high
magnetic field studies, particularly of edge states in strongly correlated
heterostructures, and the vortex state in superconducting cuprates, where RSXS
may clarify the anomalous periodicities observed in recent quantum oscillation
experiments.
