This is a complete overview of the state of the art in the study of very-low-mass stars, particularly brown dwarfs. It covers observations as well as modelling techniques, especially addressing the current status of our theoretical understanding of these objects, and the relation of observed luminosity and mass functions to star formation. Besides the problems of clustering, surface activity, and current estimates of the mass density in low-mass stars, the reader will also learn how many bona fide brown dwarfs have been unambigously detected so far.