This book provides an introduction to brown dwarfs aimed at advanced undergraduates and beginning graduate students, supplementing stellar astrophysics or (exo)planetary science classes. It emphasises the key theoretical ideas and the main observational results that support those ideas. The first chapter is an introduction to the concept of brown dwarfs, their similarities and differences with stars and planets, and classification challenges. The second section examines brown dwarf atmospheres, with an emphasis on both understanding the key concepts and making use of the detailed models. The final major section addresses the formation of brown dwarfs, the initial mass function, and the related problem of brown dwarf multiplicity. Key features: First book aimed at brown dwarfs An extension of stellar astrophysics and exoplanetary astronomy Includes Python programs Includes problems and solutions