This PhD thesis presents a study of the ferromagnetic semiconductor EuO. Chapter 1 gives a general introduction, chapter 2 presents a calculation of the magneto-optical spectrum. Chapter 3 deals with the epitaxial growth of EuO thin films by MBE. In chapter 4 experiments are used to study the spin-resolved band-structure. Chapter 5 discusses the temperature dependent X-ray absorption spectrum of EuO and chapter 6 studies its magenetic circular dichroism spectrum. Chapter 7 discusses magnetotransport and phototransport measurement and theory on EuO. Chapter 8 deals with the nature of doping in the high-Tc cuprates. This work was supervised by G.A. Sawatzky and L.H. Tjeng.