After a brief outline on the impact of massive stars for the cosmic and galactic evolution, we highlight the role of Nitrogen as a key element to test current evolutionary calculations for these objects. We recapitulate puzzling findings from the recent 'VLT-FLAMES survey of massive stars' related to Nitrogen abundance measurements in B-stars. We stress the fact that O-type stars need to be analyzed as well, and discuss corresponding difficulties arising for UV wind- and optical photospheric lines. We then concentrate on the formation of the (in-)famous Nitrogen emission lines (4634/40/42 Å) in O-stars, the so-called f-features. After numerous test calculations we found that the canonical explanation in terms of dielectronic recombination (Mihalas & Hummer, 1973, ApJ 179) is no longer or only partly valid when *modern* atmospheres including line-blocking/blanketing and winds are used. We present our current understanding about the formation of these emission lines, and finish with providing first results on the Nitrogen abundances measured from LMC.