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. The book should be of value to researchers into the evolution, structure and atmospheres of massive stars. The book is also appropriate for astrophysics courses at university level, where it can be used to get acquainted with the most recent observational data, modelling of the internal and atmospheric structure, and the refined development of single and binary evolution scenarios of massive stars.