This book presents a complete overview of what we know -- and would like to know -- about the evolution and structure of massive stars. The spectra of early type stars are produced by elaborate model atmospheres enveloping the internal structure self-consistently. The book is valuable for research on the evolution, structure and atmospheres of massive stars. Astrophysics courses at university level should use the book to get acquainted with the most recent observational data, modeling of the internal and atmospheric structure, and the refined development of single and binary evolution scenarios of massive stars.