Field ion microscopy (FIM) as well as field emission microscopy (FEM) of multiwall nanotubes (MWNTs) have been carried out to study the structure of their tips and the effect of adsorption of gas molecules on the electron emission. FIM images exhibited carbon pentagons at a nanotube tip in an atomic resolution, and also revealed changes in the tip structures under high electric field. In FEM experiments under various gas atmospheres (hydrogen, carbon monoxide, nitrogen and oxygen), adsorbates were recognized as bright spots in electron emission patterns, indicating enhanced emission of electrons through the adsorbates. Heating the MWNT emitter at about 1300K allowed adsorbates to desorb. Hydrogen and nitrogen left the intact clean caps after their desorption, while carbon monoxide damaged partially the cap structure and oxygen destroyed significantly. Finally, our recent fabrication of nanotube-based ultra-high luminance light sources which provide intense light of 10 6 cd/m 2 for green at 30kV DC-driving is reported.