Infall, accretion, and the spectroscopic evidence for planetesimals

The presence of planetesimals in protoplanetary disks can be spectroscopically detected via the presence of infalling, circumstellar gas that is episodically present, departures from the composition of the star in the infalling material, and via the presence of small, warm grains which are below the radiation-pressure blow-out radius, and hence which require rapid replenishment either via transport from the outer portions of the disk: or by the disintegration of larger bodies. The evidence for such activity is reviewed.