The presented collection of papers is based on lectures given at the International Conference "Function Spaces, Differential Operators and Nonlinear Analysis" (FSDONA-01) held in Teistungen, Thuringia/Germany, from June 28 to July 4, 2001. They deal with the symbiotic relationship between the theory of function spaces, harmonic analysis, linear and nonlinear partial differential equations, spectral theory and inverse problems. This book is a tribute to Hans Triebel's work on the occasion of his 65th birthday. It reflects his lasting influence in the development of the modern theory of function spaces in the last 30 years and its application to various branches in both pure and applied mathematics. Part I contains two lectures by O.V. Besov and D.E. Edmunds having a survey character and honouring Hans Triebel's contributions. The papers in Part II concern recent developments in the field presented by D.G. de Figueiredo / C.O. Alves, G. Bourdaud, V. Maz'ya / V. Kozlov, A. Miyachi, S. Pohozaev, M. Solomyak and G. Uhlmann. Shorter communications related to the topics of the conference and Hans Triebel's research are collected in Part III.