Low-mass stars are the dominant stellar component of the Galaxy and are important probes for understanding the Galaxy. The prolongation of the star sequence towards cooler dwarfs leads to the brown dwarfs. The transition between stellar and substellar regime is not very well known and new processes occur at these low temperatures, such as the condensation of solids in clouds. This volume offers lectures given by world experts in this field during the Evry Schatzman School on Stellar Physics (EES) of the Programme National de Physique Stellaire, held in Septembre 2011 in Roscoff, France. It gives the status of our understanding of the cool dwarfs physics on the theoretical and observational sides. The latest results are reviewed and discussed extensively. Therefore we trust this book will be a valuable reference for researchers and students in the coming years.