Vertical germanium nanowires (Figure, inset) can be synthesized on doped and undoped germanium substrates either as high-density arrays or as individual nanowires. The nanowires have smooth surfaces and uniform diameters, and possess (111) lattice fringes with an interplanar distance of 3.26 angstrom (Figure). Such vertical nanostructures are ideal for fabrication of vertical nano-transistors, such as surround-gate or top-gate transistors, and could be incorporated into other three-dimensional architectures.