  Planetesimals form in gas-rich protoplanetary disks around young stars.
However, protoplanetary disks fade in about 10 Myr. The planetesimals (and also
many of the planets) left behind are too dim to study directly. Fortunately,
collisions between planetesimals produce dusty debris disks. These debris disks
trace the processes of terrestrial planet formation for 100 Myr and of
exoplanetary system evolution out to 10 Gyr. This chapter begins with a summary
of planetesimal formation as a prelude to the epoch of planetesimal
destruction. Our review of debris disks covers the key issues, including dust
production and dynamics, needed to understand the observations. Our discussion
of extrasolar debris keeps an eye on similarities to and differences from Solar
System dust.
