We present the results of a study of the dust properties in M31. We analyzed Spitzer Space Telescope, GALEX, and SDSS images constraining some basic properties of the dust in M31: (i) the mass of the dust; (ii) the mean intensity of the radiation field heating the dust; (iii) the abundance of Polycyclic Aromatic Hydrocarbon (PAH) particles. We also studied which stellar populations are most likely responsible for the dust heating in M31 analyzing optical and infrared colors and comparing our observations with models which account for the age-dependent dust heating. We found that in most of the regions analyzed stars at least a few Gyr old are the major dust heaters. We then derived the extinction map across the disk of M31.