Abstract：Variable stars play an increasingly preponderant role in modern astronomy. They are key, for example, in the determination of cosmological distances, in probing the internal structures of stars, in the study of Galactic structure, and in the discovery of exo-planets. The advent of large-scale multi-epoch surveys since a few decades is further boosting the field. Among these surveys, ESA's Gaia mission is unique by the provision of astrometric, photometric and spectro-photometric data in a homogeneous way for more than 1.5 billion stars across the entire sky, together with spectroscopic data for the bright stars.

In this seminar, I will illustrate these considerations with the case of long-period variables. I will review their properties and their relevance for the study of the Galaxy and extra-galactic systems, and present the first catalog of these objects released by the Gaia mission in its second data release one year ago.