LISA (Laser Interferometer Space Antenna) is a low-frequency gravitational wave observatory (0.1 mHz - 1 Hz) to be launched by ESA in 2035. It aims to observe several populations of relativistic binary stars: the white dwarf binaries in our Galaxy, super-massive black holes in coalescence, stellar-mass black holes captured by super-massive black holes in galactic nuclei, etc. In addition, we hope to observe stochastic gravitational wave signals from the primordial Universe.