  We discuss the different dust components of a protoplanetary disk with a
special emphasis on grain composition, size and structure. The paper will
highlight the role dust grains play in protoplanetary disks, as well as
observational results supporting this knowledge. First, the path dust travels
from the interstellar medium into the CS disk is described. Then dust
condensation sequences from the gas are introduced, to determine the most
likely species that occur in a disk. The characteristics of silicates are
handled in detail: composition, lattice structure, magnesium to iron ratio and
spectral features. The other main dust-forming component of the interstellar
medium, carbon, is presented in its many forms, from molecules to more complex
grains. Observational evidence for PAHs is given for both young stars and solar
system material. We show how light scattering theory and laboratory data can be
used to provide the optical properties of dust grains. From the observer's
point of view, we discuss how infrared spectra can be used to derive dust
properties, and present the main spectral analysis methods currently used and
their limitations. Observational results, determining the dust properties in
protoplanetary disks, are given: first for the bright intermediate-mass Herbig
Ae/Be stars, and then for the lower-mass Tauri stars and brown dwarfs. Here we
present results from the space observatories ISO and Spitzer, as well as from
the mid-infrared interferometer VLTI, and summarise the main findings. We
discuss observational evidence for grain growth in both Herbig Ae/Be and T
Tauri stars, and its relation with spectral type and dust settling. We conclude
with an outlook on future space missions that will open new windows, towards
longer wavelengths and even fainter objects.
