  The relativistic heavy ion collider (RHIC) offers many opportunities to study
diffraction in pp, pA and AA collisions. Because both proton beams can be
polarized, RHIC offers the unique possibility of studying polarization effects
in diffraction. We will introduce diffraction at RHIC and present three
compelling physics topics: hard diffraction with polarized beams,
identification of exotic mesons (non-q q-bar states) in double-Pomeron
collisions, and using diffraction to measure the low-x gluon density in $pA$
collisions, testing models of gluon saturation and the colored glass
condensate. This note developed from discussion at a workshop on ``Diffraction
and Glueball Production at RHIC'' at Brookhaven National Laboratory, May 17-18,
2002.
