A prestigious form of research grant in Germany is the Sonderforschungsbereich, which provides continuous funding over a period of up to 15 years, but only as long as the work is yielding worthwhile results. We acknowledge financial support of our work at Erlangen by the Deutsche Forschungsgemeinschaft (DFG), Sonder- forschungsbereich 222. Thanks to this support, the experimental results from six Dr. -Ing. dissertations have provided the basis for our book: 8 * Schweinzer, J. (1987) Heat transfer in bubbling fluidized beds at Ar;a. 10 * Seiter, M. (1990) Particle motion and solids concentration in circulating fluidized beds * Mattmann, W. (1991) Heat transfer in pressurized circulating fluidized beds * Burschka, A. (1993) Pulsed light method * Dietz, S. (1994) Heat transfer in bubbling fluidized beds * Gruber, U. (1995) Heat transfer in lean phase systems This book is the result of the enthusiastic and trustful cooperation of its authors. Nevertheless, we are separate individuals. Chapters 1 to 12 and 19 are by O. Molerus; Chapters 13 to 18 are by K. -E. Wirth. This book came into existence after many rewrites, patiently endured by Mrs Winter, who typed all versions of the manuscript, and by Mrs Scheffler-Kohler, who drew all the figures. Bob Farmer and David Penfold helped us bridge the language gap to produce a readable book. Weare grateful to Professor Brian Scarlett of Delft University, who on behalf of Chapman & Hall allowed us to write this book.