High-resolution transmission electron microscopy has been used to analyze 150 nm diameter by 150 nm high polycrystalline molybdenum field-emission cones. The analysis shows that the cones comprise 5 to 10 nm thick grains with tips having gross radii of curvature of about 5 nm and protrusions having radii of curvature of about 1 nm. Such small protrusions may explain why analysis of experimental emission data indicates that the effective emission area of such tips is only 0.1 to 0.5 nm2. Furthermore, the fact that the structure is composed of small grains indicates that there is a substantial number of molybdenum atoms at grain boundaries and that many configurations of grains and boundaries are possible with minimal free energy. A qualitative model is proposed which links the structural properties to -current stabilization and hydrogen passivation effects.