This book will provide an introduction to brown dwarfs aimed at advanced undergraduates and beginning graduate students, designed to supplement stellar astrophysics or (exo)planetary science classes. The study of brown dwarfs has developed rapidly in parallel with the growth of exoplanet studies. This will continue in the 2020s with major new projects, especially the James Webb Space Telescope, which is nearly ideal for studying brown dwarf atmospheres. The main emphasis of the book will be the key theoretical ideas (physical processes) and the main observational results that support those ideas. The first chapter is an introduction to the concept of brown dwarfs, how they are similar or dissimilar to stars and planets, and the problems and benefits of classification (What is a star? What is a planet?). The second major section will be on the atmosphere of brown dwarfs. The emphasis will be on both understanding the key concepts and making use of the detailed models computed by specialists. The third and final major section addresses the formation of brown dwarfs, the initial mass function, and the related problem of brown dwarf multiplicity. Each chapter will include Python code for all figures, numerical problems with Python solutions, chapter problems with solutions.