We review the properties of dust in protoplanetary disks around optically
visible pre-main sequence stars obtained with a variety of observational
techniques, from measurements of scattered light at visual and infrared
wavelengths to mid-infrared spectroscopy and millimeter interferometry. A
general result is that grains in disks are on average much larger than in the
diffuse interstellar medium (ISM). In many disks, there is evidence that a
large mass of dust is in grains with millimeter and centimeter sizes, more
similar to "sand and pebbles" than to grains. Smaller grains (with
micron-sizes) exist closer to the disk surface, which also contains much
smaller particles, e.g., polycyclic aromatic hydrocarbons. There is some
evidence of a vertical stratification, with smaller grains closer to the
surface. Another difference with ISM is the higher fraction of crystalline
relative to amorphous silicates found in disk surfaces. There is a large
scatter in dust properties among different sources, but no evidence of
correlation with the stellar properties, for samples that include objects from
intermediate to solar mass stars and brown dwarfs. There is also no apparent
correlation with the age of the central object, over a range roughly between 1
and 10 Myr. This suggests a scenario where significant grain processing may
occur very early in the disk evolution, possibly when it is accreting matter
from the parental molecular core. Further evolution may occur, but not
necessarily rapidly, since we have evidence that large amounts of grains, from
micron to centimeter size, can survive for periods as long as 10 Myr.Comment: Protostars and Planets V in press, 16 pages, 7 figure