Doping into nanotubes is an attractive and significant way to tailor their electron transport and emission properties. This article reviews some recent experimental and theoretical advances in the studies of doping behaviors in carbon nanotubes and gallium nitrogen nanotubes, and doping effects on their field electron emission properties. The general theory for field emission mechanism of one-dimensional nanosystems is presented to provide an overall picture of the field emission process and doping behavior. Potential applications of doped nanotubes as diverse nanoscale emitters, microscopy probes, electronic guns and nanoelectronic devices are discussed.