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Microwave Plasma assisted Chemical Vapor Deposition (MPCVD) has been utilized to synthesize nitrogen doped and intrinsic nanocrystalline diamond films to investigate thermionic field emission behavior. Sulfur-doped nanocrystalline diamond films prepared by hot filament chemical vapor deposition (HFCVD) have been included in the thermionic field emission measurements. The samples were imaged in UHV by photo electron emission microscopy (PEEM) using a UV Hg lamp for photoemission excitation. The same instrument was used to obtain the thermionic-field emission electron microscopy images (T-FEEM) at temperatures up to 900°C. The Raman spectra of the films showed a strong diamond peak at 1332cm-1 and weaker signal from the graphitic regions in the sample. Field emission could not be measured at room temperature, but the PEEM images showed relatively uniform emission. The PEEM images showed little change as the temperature is increased. At temperatures as low as 640°C the T-FEEM images exhibited strongly enhanced electron emission with increasing temperature. Doped and undoped nanocrystalline diamond films showed localized emission from small emission sites with a significant temperature dependence of the electron emission for the sulfur doped films at around 600°C. This thesis focuses on developing a consistent model of thermionic emission from doped and nanocrystalline diamond films.