Observational evidence strongly favors an evolutionary relation between Wolf-Rayet (WR) stars and stars with masses above ~ 10 M⊙. The galactic distribution of WR stars closely resembles the distribution of O stars both in the z-direction as well as in the spiral-arm pattern (Hidayat, Admiranto, and van der Hucht 1984, van der Hucht et al. 1988, Conti and Vacca 1990). Lundström and Stenholm (1984) found that the surface density of W R stars increases with decreasing distance from OB associations as well as from young open clusters. The fraction of WR stars in clusters turns out to be about the same as that of O stars. A large number of extragalactic WR stars is situated in giant H II regions, which also contain high numbers of very massive 0 stars (see, e.g., Walborn 1990 for 30 Doradus; Drissen, Moffat, and Shara 1990 for M33). Many WR stars are members of binary systems with well established parameters so that their masses can be determined directly. In several cases, WR-stax masses of M ≥ 10 M⊙ have been derived (Massey 1981, St.-Louis et al. 1987, Schulte-Ladbeck 1989).