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Field emission cathodes based on carbon nanotubes are potential candidates for low-cost replacement of thermionic cathodes used in microwave tubes. This paper describes the technical approach for realizing and testing prototype field emission cathodes fabricated from aligned arrays of carbon nanotubes. The emission characteristics of prototype cathodes of up to 1 cm 2 area have been measured in both DC and pulse mode experiments. Turnon fields as low as 10 kV/cm, field enhancement factors greater than 10 3 , and emission currents higher than 5 mA have been observed. These preliminary results indicate that high current densities on the order of 1 to 10 A/cm 2 at practical electric fields are achievable with optimized cathode structure and improved degassing.