A variety of relatively transparent materials containing Eu+ + have become available, thus enabling a study to be made of the magneto-optical properties of the Eu+ + ion in widely different environments. The following materials are reviewed: Eu2SiO4 (orthorhombic structure with a ferromagnetic Curie temperature Tc of 7°K), dilute Eu+ + in CaF2 (fluorite structure), EuF2 (fluorite structure, Tc≈0), EuO−Al2O3−B2O3 glasses (Tc=0), EuSe (NaCl structure, Tc=7°K), and EuO (NaCl structure, Tc=69°K). We discuss the magneto-optical properties of the ``nonmagnetic'' materials (CaF2: Eu+ +, EuF2, Eu glasses) in terms of an intra-atomic 4f7→4f65d transition. Using this transition, calculation of the Faraday and Cotton-Mouton birefringence in EuF2 gives reasonable agreement with experiment. The magneto-optical properties of the ``magnetic materials'' (Eu chalcogenides) are discussed in terms of an interatomic (4f7) (4f7) → (4f6) (4f75d) transition. This transition gives reasonable agreement with the observed magnitudes of the magnetic absorption edge shifts in EuSe and EuO.