This special issue is dedicated to Niels Olhoff, a leading researcher and organizer in the fields of mechanics and optimization. It contains extended versions of some papers presented at a meeting celebrating Niels' 60th birthday (2nd Max Planck Workshop, Nyborg, Denmark, October 2001). Niels was born in Copenhagen, Denmark, during the Second World War in 1941. He studied at the Metropolitanskolen, obtaining his "Artium" in mathematics and physics in 1959 and his M.Sc. in Mechanical Engineering (1965). He obtained his Ph.D. in Solid Mechanics (1969) and Dr. Techn. in Optimal Engineering Design (1979) at the TU Denmark in Lyngby. His major research interests are Solid Mechanics, Structural Vibration and Stability, Computational Mechanics, Acoustics and Fluid-Solid Interactions, Computer-Aided Engineering Design and Multidisciplinary Design Optimization. Niels' professional career included military service in the Danish Navy (1965-67), followed by academic activities at the TU Denmark, where he started as a Ph.D. student but soon reached the position of Associate Professor. Since 1985 he has been a full professor at Aalborg University. He has received numerous awards and honours, amongst many others the Esso Prize, Max Planck Research Award, and membership of the Danish Academy of Technical Sciences and of the Royal Society of Arts and Sciences in Gothenburg, Sweden. He was recently decorated as Knight of Dannebrog by the Queen of Denmark.