This unique volume is devoted to the life and scientific work of the distinguished Swedish theoretical physicist Gunnar Källén (19261968). In spite of his short life span, Källén had a huge impact on the scientific lives of a large number of young scientists. His sharp intellect and exceptional talent to see through complicated structures and quickly pinpoint the essentials were legendary, as were also his outspokenness and strong opinions. No one who ever met him was left indifferent.Källén is well known for his work on quantum electrodynamics; for his work with Pauli on the mathematical structure of the Lee Model; and for his work with Arthur Wightman on the analyticity domain of the vacuum expectation value of product of three scalar fields. He is also remembered by many for his textbook on Elementary Particle Physics.A young man at Källén's time, who in this volume expresses his "great debt of gratitude" to him, is the 1979 Nobel Laureate in Physics Steven Weinberg. His 2009 Källén Memorial Lecture "Living with Infinities" was written specifically for this volume.The 1957 Nobel Laureate in Physics T D Lee provides a historical perspective on his model (Lee Model, 1954) that was studied by many scientists, including Källén and Pauli.Other contributors to this book include Arberto Sirlin on Källén's work on radiative corrections in weak interactions; Raymond Stora and Stephen Gasiorowicz on Källén's work in quantum electrodynamics. In addition, the Editors provide commentary on some of Källén's scientific papers.Gunnar Källén's childhood and youth are described by his brother, Bengt Källén and his tragic death in a plane crash by his son Erland Källén. Other contributors reflect upon their interactions with Gunnar Källén.