  Intensity mapping has been attracting increasing interest as a way to study
galaxy evolution and the large scale structure of the Universe. Instead of
detecting individual galaxies, we measure the integrated emission from a volume
of the universe. Contrary to galaxy surveys, it includes light from the
faintest objects in the cosmic web. We will start this overview by introducing
line intensity mapping and review its current observational status. We will
enumerate the most prominent emission lines for intensity mapping and discuss
how to model their observed signal. The prospects of using this technique in
the near future are very good with a wide range of experiments in different
parts of the electromagnetic spectrum. As for any other observational tool,
intensity mapping is not free of systematic uncertainties, some of which are
intrinsic to it. We will briefly discuss them as well as the means to deal with
foreground and background contamination. Here, we will focus on the science
cases for intensity mapping in the post-reionization universe. These range from
the history of star formation to measuring the acoustic scale and the
fundamental physics.
