I made my PhD at the Ludwig-Maximilian University, in Munich, creating more realistic atmosphere models for massive stars, by using novel radiative transfer techniques. My new observations in the Galaxy led me to present the so-called mass discrepancy, a problem with a high impact on the community, and to explore the physics of stellar winds. Both have been activity engines in the field. Later, my research extended to other galaxies and to obscured clusters in our Galaxy. In the former, my team determined for the first time the stellar O abundance gradient in a nearby galaxy (M33), the abundances in B-supergiants beyond the Local Group (NGC 300), and extended the study of massive stars to low metallicity galaxies (IC 1613, Sex A) in an effort to near the early Universe conditions. As part of it, I have participated in international collaborations based on ESO large programmes (FLAMES Survey of massive stars, VLT-FLAMES Tarantula Survey) now continued by the XShootU (based on an ESO large programme plus a HST legacy project) and BLOeM collaborations. In the