"Meditationis est perscrutari occulta; contemplationis est admirari perspicua . . . Admiratio generat qurestionem, qurestio investigationem, investigatio inventionem" once wrote HUGO DE S. VICTORE and possibly described best the life span of the rare earths since the Swedish Army Lieutenant, C. A. ARRHENIDS' discovery of an unusual black mineral at Ytterby in 1787. Since then the rare earths have passed through the machinery of various phases of physical science. Europium as a member of the rare earth family shows characteristics of the series as a whole as well as some typical behaviour of its own due to the presence of its six f electrons i. e. , one less than the half-filled shell. This present volume attempts to provide an understanding of the various aspects of its chemical and physical behaviour. In doing so, it is hoped to provide the reader with more than just a list of the complete bibliography on the subject, and I have tried to review the literature for it's interesting features, balancing the experimental facts with the help of the minimum amount of theory as far as possible. Experience has shown us that the trend of inorganic research has changed during the past years and one frequently needs some understanding of chemical spectroscopy and quantum mechanics to be able to appreciate the developments in this field and to interpret the wealth of experimental data. The reader is referred to B. G. WYBOURNE'S book: Spectroscopic Properties of Rare Earths (ref. [488]), and B. R.