  We present a new model of interstellar dust in which large grains are a
single composite material, ``astrodust,'' and nanoparticle-sized grains come in
distinct varieties including polycyclic aromatic hydrocarbons (PAHs). We argue
that a single-composition model for grains larger than $\sim$0.02 $\mu$m most
naturally explains the lack of frequency dependence in the far-infrared (FIR)
polarization fraction and the characteristic ratio of optical to FIR
polarization. We derive a size distribution and alignment function for 1.4:1
oblate astrodust grains that, with PAHs, reproduce the mean wavelength
dependence and polarization of Galactic extinction and emission from the
diffuse interstellar medium while respecting constraints on solid phase
abundances. All model data and Python-based interfaces are made publicly
available.
