Turneaure, J.P. JACoW Publishing Geneva, Switzerland This talk will start with a brief discussion of why superconducting Nb was of interest for particle accelerator structures. The results of the first Nb cavities that achieved high Q₀¿s up to 10¹¹ and peak electric and magnetic fields of 70 MV/m and 1080 Oe will be described. Then the approach to producing L-band and S-band prototype TM010 mode cavities and accelerator structures will be described, followed by summary results and what was learned from the tests on these cavities and structures. Finally, electron-field-emission loading and electron multipacting experiments and their results will be described.