  Electrical transport through a normal metal / superconductor contact at
biases smaller than the energy gap can occur via the reflection of an electron
as a hole of opposite wave vector. The same mechanism of electron-hole
reflection gives rise to low energy states at the surface of unconventional
superconductors having nodes in their order parameter. The occurrence of
electron-hole reflections at normal metal / superconductor interfaces was
predicted independently by Saint James and de Gennes and by Andreev, and their
spectroscopic features discussed in detail by Saint James in the early sixties.
They are generally called Andreev reflections but, for that reason, we call
them Andreev - Saint James (ASJ) reflections. We present a historical review of
ASJ reflections and spectroscopy in conventional superconductors, and review
their application to the High $T_c$ cuprates. The occurrence of ASJ reflections
in all studied cuprates is well documented for a broad range of doping levels,
implying that there is no large asymmetry between electrons and holes near the
Fermi level in the superconducting state. In the underdoped regime, where the
pseudo-gap phenomenon has been observed by other methods such as NMR, ARPES and
Giaever tunneling, gap values obtained from ASJ spectroscopy are smaller than
pseudo-gap values, indicating a lack of coherence in the pseudo-gap energy
range.
