  We present a critical discussion of recent experimental probes of the pairing
state of the high temperature superconductors, focusing primarily, but not
exclusively, on \Yba, where the best data currently exist. Penetration depth
measurements near \Tc\ give no indication of an extra transition, indicating
that the pairing state is a one-dimensional representation of the crystal
symmetry. Penetration depth measurements at low temperatures provide strong
evidence for a change in sign of the gap function over the Fermi surface.
Quantum mechanical phase interference experiments generally confirm this and in
addition show that the nodal positions are consistent with a \dx2y2\ pairing
state. This pairing state is consistent with photoemission measurements of the
gap function, Raman scattering, the effect on \Tc\ of impurities, and many
other data (reviewed by two of us previously) which indicate the presence of
low lying excitations in the superconducting state. We also discuss evidence
that apparently does not fit in with a \dx2y2\ pairing state, and we describe
possible alternative scenarios.
