Twinning in metallic materials. The formation and growth of twins result in substantial evolution of microstructures and properties in a variety of metallic materials. The articles in this issue of MRS Bulletin overview the synthesis and mechanical behavior of nanotwinned metallic materials as well as plasticity dominated by mechanical twinning. The cover shows a transmission electron microscope micrograph of sputterdeposited epitaxial nanotwinned Ag in a cross-sectional view examined from the [01$\overline 1 $] zone axis. The image reveals highdensity nanoscale growth twins with coherent twin boundaries normal to the growth direction. The average twin spacing is ~9 nm, and the average columnar domain size is ~120 nm. Image courtesy of Daniel Bufford, Sandia National Laboratories, and Xinghang Zhang, Texas A﹠M University. The image was taken at the Microscopy and Imaging Center at Texas A﹠M University. The research on nanotwinned Ag was funded by NSF-DMR Metallic Materials and Nanostructures Program, under Grant No. 0644835. See the technical theme that begins on page 274.