Ninety years have ellapsed since the Old Quantum Theory has emerged, and eighty three over the foundations of Modern Quantum Mechanics. Born in 1901, Ruy Gustavo Couceiro da Costa soon became aware of the importance of Quantum Mechanics in Science, particularly in Chemistry. Such a vision has flurished ever since and its presence in the scientific realm is nowadays unquestionable: Physics, Chemistry, Biology, Astronomy, Engineering and even Philosophy, all such areas of knowledge reflect the importance of judgement in accordance with the quantum laws. This book is a result of a Symposium to honor the memory of Professsor Couceiro da Costa for his contribution to the development of Quantum Mechanics in Chemistry and Physics in Portugal.