Giant planets can be up to 13 times the mass of Jupiter, while the least massive stars are about 80 times the mass of Jupiter. In between are objects called “brown dwarfs” – too massive to be called planets, but not massive enough to burn hydrogen and shine like stars. Since 1994, a few thousand brown dwarfs have been observed close to us in the galaxy. But, what are they? Are they more like half-pint cousins of stars, or more like overgrown planets? This lecture will explain how we observe and study brown dwarfs and what we have learned about them. It will describe clues to their nature from their composition and their evolution over time, and the insights they give us into how stars and planets are born.