These substellar objects were first conceived of in the early 1960s as “failed stars”. Stellar models suggested that a true star must have a mass at least 80 times that of Jupiter to kindle the stable fusion of hydrogen (this limit is currently thought to be about 75 jupiters). Objects with less mass were believed to exist, but it was recognized that they would be extremely difficult to find because they would be quite low in luminosity, and emit very little visible light. For that reason, they can be viewed as a constituent of baryonic “dark matter”. Indeed, we currently have a hard time directly seeing an old brown dwarf beyond 100 pc.
After 20 years of searching and false starts, the first confirmed brown dwarfs were announced in 1995. This was due to a combination of increased sensitivity, better search strategies, and new means of distinguishing substellar from stellar objects. Since then, a great deal of progress has been made on the observational front. We are now in a position to say a substantial amount about actual brown dwarfs. We have a rough idea of how many of them occur as solitary objects and how many are found in binary systems.
We have obtained the first glimpse of atmospheres intermediate in temperature between stars and planets, in which dust formation is a crucial process. This has led to the proposal of the first new spectral classes in several decades, and the need for new diagnostics for classification and setting the temperature scale. The first hints on the substellar mass function are in hand, although current masses depend on models. It appears that numerically, brown dwarfs may be nearly as common as stars (though they do not contain a dynamically interesting amount of mass).