  This paper reviews the theory of protostellar debris disks. After a brief
introduction to accretion disk physics, I describe coagulation models of planet
formation in the outer regions of planetesimal disks. Coagulation models for
the Kuiper Belt produce Pluto-sized objects on timescales of 10-40 Myr. These
models yield size distributions which agree with observations of Kuiper Belt
objects with red magnitudes, R = 20-27. Velocity stirring models for other
debris disk systems demonstrate that 500 km or larger objects can stir the
velocities of small objects up to the shattering limit needed to begin a
collisional cascade.
