  A question central to understanding the origin of our solar system is: how do
planets form in circumstellar disks around young stars? Because of the complex
nature of the physical processes involved, multi-wavelength observations of
large samples will be required in order to obtain a complete answer to this
question. Surveys undertaken with ISO have helped to solve pieces of this
puzzle in addition to uncovering new mysteries. We review a variety of studies
aimed at understanding; i) the physical structure and composition of
circumstellar disks commonly found surrounding young stellar objects; and ii)
the evolution of circumstellar disks from the active accretion phase to
post-planet building debris disks.
