  The Wolf-Rayet (WR) phenomenon is widespread in astronomy. It involves
classical WRs, very massive stars (VMS), WR central stars of planetary nebula
CSPN [WRs], and supernovae (SNe). But what is the root cause for a certain type
of object to turn into an emission-line star? In this contribution, I discuss
the basic aspects of radiation-driven winds that might reveal the ultimate
difference between WR stars and canonical O-type stars. I discuss the aspects
of (i) self-enrichment via CNO elements, (ii) high effective temperatures Teff,
(iii) an increase in the helium abundance Y, and finally (iv) the Eddington
factor Gamma. Over the last couple of years, we have made a breakthrough in our
understanding of Gamma-dependent mass loss, which will have far-reaching
consequences for the evolution and fate of the most massive stars in the
Universe. Finally, I discuss the prospects for studies of the WR phenomenon in
the highest redshift Ly-alpha and He II emitting galaxies.
