Observations of Galactic dust are a highlight and a lasting legacy of the Planck space mission. Spectacular images combining the intensity of dust emission with the texture derived from polarization data have received world-wide attention and become part of the general scientific culture. Many more know today that the Galaxy is magnetized. Beyond their popular impact, the Planck dust maps are an immense step forward for Galactic astrophysics, greatly superseding earlier observations. First, they have provided astrophysicists with the data they need to statistically characterize the structure of the Galactic magnetic field and its coupling with interstellar matter and turbulence, in the diffuse interstellar medium and star-forming molecular clouds. Second, Planck multi-frequency observations have opened a new perspective on interstellar dust, unveiling its sub-mm polarization properties. Last, with Planck, dust polarization has become inter-connected to a paramount objective of observational cosmology: the quest for curl-like(B-mode) polarization of the cosmic microwave background expected from primordial gravitational waves. Today, the component-separation challenge binds the search for primordial B modes to the astrophysics of the magnetized interstellar medium. I will introduce these science topics and highlight key results.