  In this talk I first discuss the experimental evidence for multiple
scattering and the properties of the underlying event. The extensive analyses
by Rick Field of data from CDF cannot be reconciled with traditional wisdom
concerning multiple collisions and the AGK cutting rules. Data seem to imply
some kind of color recombination or unexpectedly strong effects from pomeron
vertices. I then discuss theoretical ideas concerning the relation between
multiple collisions and unitarity: the AGK rules, IP loops, dipole cascade
models and diffraction.
