Processes for growing carbon nanotubes on carbon fiber substrates are described herein. The processes can include depositing a catalyst precursor on a carbon fiber substrate, optionally depositing a non-catalytic material on the carbon fiber substrate, and after depositing the catalyst precursor and the optional non-catalytic material, exposing the carbon fiber substrate to carbon nanotube growth conditions so as to grow carbon nanotubes thereon. The carbon nanotube growth conditions can convert the catalyst precursor into a catalyst that is operable for growing carbon nanotubes. The carbon fiber substrate can remain stationary or be transported while the carbon nanotubes are being grown. Optionally, the carbon fiber substrates can include a barrier coating and/or be free of a sizing agent. Carbon fiber substrates having carbon nanotubes grown thereon are also described.