I review our knowledge about effective temperatures and masses of massive OB stars paying special attention to results recently obtained using new atmospheric models including sphericity, mass-loss and line-blanketing. The new temperature scales for O dwarfs and supergiants are lower than previously accepted. No systematic mass discrepancy is found among O supergiants, although individual problematic cases still remain. It is shown that model atmospheres give answers consistent with binary systems. Good agreement is found between theoretical predictions and observations of the Wind Momentum-Luminosity relationship for Galactic O-type supergiants. Finally, I review the status of the WLR in M 33.