Abstract　 　　 

    The detailed analysis of colour-magnitude diagrams obtained at the Canada-France-Hawaii telescope has revealed hitherto unknown properties of the nearest spiral galaxy, M31. The recent star formation history in the disc shows that M31 is a starving galaxy in the so-called green valley. Time-domain series carried out over two years provide the first comprehensive variability inventory of this galaxy. Hubble Space Telescope follow-up observations of some of the Cepheids discovered have led to a determination of its distance with an error of 3%, and likewise for the Hubble constant, by-passing the uncertainties in the calibration of the Magellanic Clouds. The analysis of the halo fields has revealed not only many substructures at faint surface brightness levels, but a wealth of information on new satellites, globular clusters, stellar streams, and the structure of the halo. Over half of the satellites of M31 appear to lie in a vast disc, which is rotating around the galaxy, yielding stringent constraints on the accretion mechanisms which lead to the formation of galaxies.