Massive stars play a vital role in shaping the cosmic matter cycle and driving galaxy evolution, chemically enriching their host galaxy through their powerful stellar winds. Understanding the physical processes behind these mass-loss events is key to producing accurate model predictions. Despite its importance, stellar atmosphere modelling poses several challenges. In this talk, we will explore these challenges accross a range of massive stellar types, from OB main-sequence stars to evolved red supergiants and Wolf-Rayets. With these insights, I will further discuss the consequences of mass outflows on stellar evolution and the surrounding environment.