Massive stars play a crucial role in the Universe: they are important drivers for the photometric and chemical evolution of galaxies; they are sources of important elements, including those necessary for life; and, with their strong winds and supernova explosions, they feed the interstellar medium with momentum and kinetic energy, impacting on the star formation rate. Knowledge of the evolution of massive stars is important not only for stellar physics, but also for probing the evolution of galaxies and their star formation histories throughout cosmic time. This volume provides an introduction to these topics and to the techniques used to investigate the properties of massive stars, including asteroseismology, spectropolarimetry, and interferometry. It highlights synergies between these new techniques and more classical methods, to create a synthetic view of massive stars, leading researchers towards new and innovative solutions to the most topical questions regarding the evolution of massive stars.