Massive stars are distributed all over the upper part of the Hertzsprung-Russell diagram according to their subsequent phases of stellar evolution from main sequence to supernova. Massive stars may either be single or they may be a component of a close binary. The observed single star/binary frequency is known only in a small part of the Galaxy. Whether this holds for the whole galaxy or for the whole cosmos is questionable and needs many more high quality observations. Massive star evolution depends critically on mass loss by stellar wind and this stellar wind mass loss may change dramatically when stars evolve from one phase to another. We start the book with a critical discussion of observations of the different types of massive stars, observations that are of fundamental importance in relation to stellar evolution, with special emphasis on mass loss by stellar wind. We update our knowledge of the physics that models the structure and evolution of massive single stars and we present new calculations. The conclusions resulting from a comparison between these calculations and observations are then used to study the evolution of massive binaries. This book provides our current knowledge of a great variety of massive binaries, and hence of a great variety of evolutionary phases. A large number of case studies illustrates the existence of these phases. Finally, we present the results of massive star population number synthesis, including the effect of binaries. The results indicate that neglecting them leads to a conclusion which may be far from reality. This book is written for researchers in massive star evolution. We hope that, after reading this book, university-level astrophysicsstudents will become fascinated by the exciting world of the Brightest Binaries'.