This book records our current understanding of the observational and theoretical properties of objects known as ultracool dwarfs. It covers the state of the art in this new field. It is split into theoretical, observational and spectral classification sections. Each subject area begins with an introduction by an eminent scientist. It covers a wide range of issues, such as the transition from L to T dwarfs, dust and alkali metal modelling, companions, activity, the deuterium test, and brown dwarf variability, and contains considerable discussion about spectral classification schemes. The articles arose from an IAU meeting and they address researchers as well as graduate students.