  The wavelength dependences of interstellar extinction and polarization,
supplemented by observed elemental abundances and the spectrum of infrared
emission from dust heated by starlight, strongly constrain dust models. One
dust model that appears to be consistent with observations is presented. To
reproduce the observed extinction, the model consumes the bulk of interstellar
Mg, Si, and Fe (in amorphous silicates), and a substantial fraction of C (in
carbonaceous material), with size distributions and alignment adjusted to match
observations.
  The composition, structure, and size distribution of interstellar grains is
the result of injection of dust from stellar outflows into the interstellar
medium (ISM), followed by destruction, growth, coagulation, and photoprocessing
of interstellar grains. The balance among these poorly-understood processes is
responsible for the mix of solid material present in the ISM. Most interstellar
grain material present in the diffuse ISM must be grown in the ISM. The
amorphous silicate and carbonaceous materials that form the bulk of
interstellar dust must therefore be the result of grain growth in the presence
of ultraviolet radiation. Dust in high-z systems such as J1148+5251 is also
produced primarily in the ISM, with supernova-produced dust contributing only a
small fraction of the total dust mass.
