This Festschrift volume in honour of Prof. E R Caianiello contains invited papers of eminent scientists who have worked in the several areas to which Prof. Caianiello has given seminal contributions: quantum field theory, foundations of quantum mechanics and maximal acceleration (Vol. 1); neural nets, general systems theory and various topics of cybernetics (Vol. 2). The wide range of topics covered shows the fruitfulness of a higher unifying perspective on seemingly diverse subjects.

This Festschrift volume in honour of Prof. E R Caianiello contains invited papers of eminent scientists who have worked in the several areas to which Prof. Caianiello has given seminal contributions: quantum field theory, foundations of quantum mechanics and maximal acceleration (Vol. 1); neural nets, general systems theory and various topics of cybernetics (Vol. 2). The wide range of topics covered shows the fruitfulness of a higher unifying perspective on seemingly diverse subjects.

This volume is about "Structure". The search for "structure", always the pursuit of sciences within their specific areas and perspectives, is witnessing these days a dra matic revolution. The coexistence and interaction of so many structures (atoms, hu mans, cosmos and all that there is in between) would be unconceivable according to many experts, if there were not, behind it all, some gen eral organizational principle. s that (at least in some asymptotic way) make possible so many equilibria among species and natural objects, fan tastically tuned to an extremely high degree of precision. The evidence accumulates to an increasingly impressive degree; a concrete example comes from physics, whose constant aim always was and is that of searching for "ultimate laws", out of which everything should follow, from quarks to the cosmos. Our notions and philosophy have un dergone major revolutions, whenever the "unthinkable" has been changed by its wonderful endeavours into "fact". Well, it is just from physics that evidence comes: even if the "ultimate" could be reached, it would not in any way be a terminal point. When "complexity" comes into the game, entirely new notions have to be invented; they all have to do with "structure", though this time in a much wider sense than would have been understood a decade or so ago.

Wolfgang Pauli referred to him as 'my discovery,' Robert Oppenheimer described him as 'one of the most gifted theorists' and Niels Bohr found him enormously stimulating. Who was the man in question, Gunnar Källén (1926-1968)? His appearance in the physics sky was like a shooting star. His contributions to the scientific debate caused excitement among young and old. Similar to his friend and mentor, Wolfgang Pauli, he demanded honesty and rigor in physics - a distinct dividing line between fact and speculation. In his obituary, Arthur S. Wightman would write: 'Gunnar Källén was a proud continuer of the tradition in quantum field theory established by Wolfgang Pauli. His papers on quantum electrodynamics in the period 1950-1954 carried the non-perturbative approach to quantum electrodynamics forward to a point beyond which very little essential progress has been made up to the present day. At the time I was trying to puzzle out the grammar of the language of quantum field theory, and here was Källén already writing poetry in the language!'. In addition to being a remarkable scientist, Källén had a very interesting personality, well worth exploring. In her book, physicist Cecilia Jarlskog traces both the personal and scientific trajectory of this unsung hero of the early days of high-energy physics and quantum field theory. A number of invited contributions by members of the Källén family and distinguished researchers from the field, all of them personally acquainted with Källén, combine to form an authentic portrait of the researcher and the man. Last but not least, the reader will become acquainted with some aspects of the history of particle physics in those days, as related by Källén and those who corresponded with him. A commented selection of his most important and not easily accessible papers is included as an added bonus for specialists.