Vertically aligned carbon nanotubes and nanofibers are grown on metallic Ta and Pd underlayers at temperatures below 500°C. Controlling the size of the grains of the underlayer film is critical because this leads to a more uniform distribution of catalyst dots, which in turn results in vertical alignment of the carbon nanostructures. Rapid and limited heating and appropriate materials selection can also be used to limit catalyst/underlayer reactions that hinder or suppress carbon nanostructure growth or that lead to entangled growth. Control of catalyst reactivity with metallic underlayers is significant because growth on conductive substrates is notoriously difficult, but needed for many applications such as the use of carbon nanostructures in microelectronic circuits. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA.