The evolution of massive stars: a selection of facts and questions

In the present paper we discuss a selection of facts and questions related to observations and evolutionary calculations of massive single stars and massive stars in interacting binaries. We focus on the surface chemical abundances, the role of stellar winds, the early Be-stars, the high mass X-ray binaries, the effects of rotation on stellar evolution. Finally, we present an unconventionally formed object scenario (a UFO-scenario) of WR binaries in dense stellar environments.