  We review the description of high energy diffraction from both the s and t
channel viewpoints, and demonstrate their consistency. We emphasize the role
played by s channel unitarity and multi-Pomeron exchanges. We explain how these
effects suppress hard diffractive processes. As examples, we describe the
calculation of diffractive dijet production at the Tevatron and predict some of
the rich diffractive phenomena accessible in proton-nuclear collisions at RHIC.
