Protoplanetary disks are the birth places of planetary systems. Most
young stars are surrounded by such planet forming disks. The onset of
the processes associated with planet formation significantly alter the
radial and vertical structure of the disks, modify the gas and dust
content in them, and cause grain growth and crystallisation of the
dust in the disks. Observationally, these structural and evolutionary
changes are best studied at infrared wavelengths. As part of a large
Spitzer Infrared Spectrograph (IRS) survey, we have obtained and
analysed mid-infrared (5-40 micron) spectra of several hundred young
protoplanetary disks in the nearby star forming regions. I will
present the main results of this survey and discuss their implications
for our understanding of the early stages of planet formation and the
evolution and eventual dissipation of protoplanetary disks.