This volume, written by a highly cited author, presents the history of quantum theory together with open questions and remaining problems in terms of the plausibility of quantum chemistry and physics. It also provides insights into the theory of matter-wave mechanics. The content is aimed at students and lecturers in chemistry, physics and the philosophy of science.

Born in South Africa in 1934, Jan Boeyens started his research career in Chemistry and Physics at the SA National Laboratories, Pretoria, South Africa and then moved on abroad to work in several prestigious universities, notably:- Stanford University, California, USA (Research Associate)- University of the Witwatersrand, Johannesburg, South Africa (Professor of Theoretical Chemistry and Dean of Science)- Texas A& M University, USA (Visiting Professor)- Free University of Berlin, Germany (Guest Professor)- Ruprecht Karls University, Heidelberg, Germany (Regular Visiting Professor)- University of Pretoria, South Africa (Fellow: Unit for Advanced Study)In 1993, the Alexander von Humboldt Foundation (Germany) awarded him with their Research Prize for Theoretical Chemistry. During his research career, he has also received several South-African Research Awards.From 1996 to 2002, he was an executive member of the International Union of Crystallography.He has authored 275 refereed papers in Crystallography and Structure Theory, edited several Specialist Proceedings and has five books on Theoretical Chemistry and Cosmology to his credit. Prof. Jan Boeyens passed away in Pretoria, August 2015.

The Mysterious Quantum.- The Cosmological Debate.- What Happened ....- The Classical Basis.- Delicious Mysteries.- Non-Classical Phenomena.- Non-linear Phenomena.- Space, Time and Matter.- Matter-wave Mechanics.- Epilogue.- Quaternion Rotations.- Separation of the Variables.- Broken Gauge Symmetry.- Projective Geometry.- Nucleogenesis.