Pixel lensing: Microlensing towards M31

Pixel lensing is gravitational microlensing of unresolved stars. The main target explored up to now has been the nearby galaxy of Andromeda, M31. The scientific issues of interest are the search for dark matter in form of compact halo objects, the study of the characteristics of the luminous lens and source populations and the possibility of detecting extra-solar (and extra-galactic) planets. In the present work we intend to give an updated overview of the observational status in this field.Comment: Invited article for the GRG special issue on gravitational lensing (Ph. Jetzer, Y. Mellier and V. Perlick Eds.