This book contains the proceedings of an August 2000 conference held in honor of one of the founders of the theory of interpolation spaces. Contributions cover a range of topics both from interpolation theory and from other fields where Jaak Peetre's ideas have had an impact, including the theory of function spaces, Hankel-type and related operators, analysis on bounded symmetric domains, and special functions. The book opens with biographical material and a list of Peetre's publications, followed by his paper on the birth of the theory of interpolation, plus a paper from the late co-founder of this theory.