The observed interstellar extinction is reviewed briefly. A dust model consisting of amorphous silicate grains + carbonaceous grains (including PAHs) can successfully reproduce the observed extinction on different sightlines given appropriate size distributions. The model grains, exposed to interstellar starlight, will radiate in the infrared and far-infrared. The expected emission can be calculated from the physical properties of the grains. Grains in the SMC differ substantially from Milky Way dust in their extinction properties, and the calculated infrared emission spectra also differ. Recent observations of infrared emission from the Sb galaxy NGC 7331 are compared to the emission calculated for the amorphous silicate + carbonaceous grains dust model, using the size distributions adopted previously for modeling the dust in the local diffuse interstellar medium of the Milky Way. With appropriate choice of illuminating radiation field, the model reproduces the observed infrared emission, with the exception of a newly-discovered broad emission feature at 17.1μm. The 17.1μm feature, with FWHM = 0.96μm, carries approximately 3 times as much power as the 16.4μm PAH feature.
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