Metabolic engineering is a rapidly evolving field that is being applied for the optimization of many different industrial processes. In this issue of Advances in Biochemical Engineering/Biotechnology, developments in different areas of metabolic engineering are reviewed. The contributions discuss the application of metabolic engineering in the improvement of yield and productivity - illustrated by amino acid production and the production of novel compounds - in the production of polyketides and extension of the substrate range - and in the engineering of S. cerevisiae for xylose metabolism, and the improvement of a complex biotransformation process.