Brown dwarfs and exoplanets are a hot topic in astrophysics. Brown dwarfs are substellar objects not massive enough to burn hydrogen, which hence cool with time. Exoplanets refer to even lower mass bodies, with a mass smaller than a dozen of Jupiter masses. The past decade has witnessed the routine detections of the first brown dwarfs and the discovery of 51 Peg B, the first exoplanet (1995). Several hundreds of such objects have now been discovered using different techniques and are intensively studied. As we learn more about them, more problems arise, and unexpected characteristics are unveiled. How do brown dwarfs and exoplanets form? Is there weather on brown dwarfs, some of which being covered by clouds of iron and refractory elements? Is our closest neighbour a brown dwarf? Do the currently known planets draw a picture similar to our own Solar system? When and how will we see the first exoplanet?

In this course we will cover 1) the history of brown dwarfs and exoplanets: theoretical prediction, contribution of the former to the Galactic mass budget; 2) how these objects were first discovered and what the current and planned search programs are; 3) the nature of the substellar objects, and taxinomy; 4) their internal structure and their atmosphere; 5) how we currently explain the formation of brown dwarfs and exoplanets.

We will balance theory (structure and atmospheric modeling, formation scenarii) and discussion of observations (methods, direct and indirect detections, observatories and instrumentation, practical problems and limitation, prospects).