  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.
