Summary: Drawing on the observations presented in Chapter 1, Chapter 4 presents new models of interstellar dust that reproduce simultaneously current observations of dust extinction and emission, both total and polarized. Particular attention is paid to the frequency dependence of the polarized infrared emission, which is found to be compatible with both the presence of ferromagnetic nanoparticles embedded in silicate grains or magnetic dipole emission in large, highly conducting graphitic grains. Chapter 4 concludes by discussing observational predictions of these models and their possible extensions beyond the diffuse interstellar medium.