Protoplanetary Disks and Their Evolution
  Flattened, rotating disks of cool dust and gas extending for tens to hundreds
of AU are found around almost all low mass stars shortly after their birth.
These disks generally persist for several Myr, during which time some material
accretes onto the star, some is lost through outflows and photoevaporation, and
some condenses into centimeter- and larger-sized bodies or planetesimals.
Through observations mainly at infrared through millimeter wavelengths, we can
determine how common disks are at different ages, measure basic properties
including mass, size, structure, and composition, and follow their varied
evolutionary pathways. In this way, we see the first steps toward exoplanet
formation and learn about the origins of the Solar System. This review
addresses observations of the outer parts, beyond 1 AU, of protoplanetary disks
with a focus on recent infrared and (sub-)millimeter results and an eye to the
promise of new facilities in the immediate future.
