  The recent discovery of a gravitational wave from the merging of two black
holes of about 30 solar masses each challenges our incomplete understanding of
massive stars and their evolution. Critical ingredients comprise mass-loss,
rotation, magnetic fields, internal mixing, and mass transfer in close binary
systems. The imperfect knowledge of these factors implies large uncertainties
for models of stellar populations and their feedback. In this contribution we
summarize our empirical studies of Wolf-Rayet populations at different
metallicities by means of modern non-LTE stellar atmosphere models, and
confront these results with the predictions of stellar evolution models. At the
metallicity of our Galaxy, stellar winds are probably too strong to leave
remnant masses as high as 30 solar masses, but given the still poor agreement
between evolutionary tracks and observation even this conclusion is debatable.
At the low metallicity of the Small Magellanic Cloud, all WN stars which are
(at least now) single are consistent with evolving quasi-homogeneously. O and
B-type stars, in contrast, seem to comply with standard evolutionary models
without strong internal mixing. Close binaries which avoided early merging
could evolve quasi-homogeneously and lead to close compact remnants of
relatively high masses that merge within a Hubble time.
