This book will present the first comprehensive discussion of both the astrophysical structure of individual M-dwarf and brown dwarf star, and their collective statistical properties as a Galactic stellar population. The first section of the book will discuss M-dwarfs and brown dwarfs as individual objects - their observational properties, formation, internal structure and atmospheres. The second section will deal with M-dwarfs from the Galactic perspective - the number of stars, their possible contribution to dark matter and the missing mass and their use as probes of the stellar populations that make up our Galaxy.