Carbon nanotubes (CNTs) have been synthesized in a continuous-flow, gas-phase catalytic process. The synthesis technique consists of two steps: 1) production of well-defined metal nanoparticles in an atmospheric-pressure microplasma and 2) catalytic growth of carbon nanotubes in a tube furnace reactor. In the first step, nanoparticles are generated using a direct-current (dc) hollow cathode microplasma made-up of a stainless steel cathode with a pin-hole (d~180 microns) and an arbitrarily-shaped tube anode. Gaseous precursors are introduced into the microplasma at atmospheric-pressure and decomposed non-thermally by electron impact to generate reactive radical species. Under appropriate precursor saturation conditions, the radicals polymerize to nucleate particles homogenously in the gas phase. Particle growth is limited to the small reactor volume (less than 1 nanoliter) created by the microplasma geometry. As a result of the large concentration gradients and short residence time, the technique is capable of producing very small (1-3 nanometer diameter) nanoparticles with narrow size distributions. The particle-laden flow is then continuously fed to a second reactor to grow carbon nanotubes in free flight with addition of acetylene and hydrogen and heating at fixed temperatures between 500 and 1000 degrees Celsius. Nanotube size and distribution are determined on line using a gas-phase electrophoretic mobility macromolecular analyzer (GEMMA). In situ aerosol classification allows experimental conditions to be directly related to growth parameters. We have recently investigated the catalytic properties of iron and nickel nanoparticles toward growth of carbon nanotubes. Process parameters were optimized to prevent amorphous carbon formation and obtain high-quality CNTs. Ex-situ techniques such as Raman spectroscopy and transmission electron microscope (TEM) were used to characterize the structure and geometry of the carbon nanotubes. The combination of continuous-flow synthesis using microplasmas and the GEMMA system opens new possibilities for nanomaterial synthesis and provides a methodology for enhancing our fundamental understanding of catalytic behavior.