The　development　of　high　current-density　cathodes　employing　scandia-doped　tungsten　powders　is　reviewed　in　this　paper.　A　matrix　with　a　submicrometer　microstructure　characterized　by　uniformly　distributed　nanometer　particles　of　scandia　is　believed　to　play　a　dominant　role　in　the　improved　emission　capability　of　these　cathodes.　Space-charge-limited　current　densities　of　over　30　A/cm(2)　at　850　degrees　C-b　have　been　repeatedly　obtained　for　many　runs　of　cathodes　fabricated　from　the　different　batches　of　scandia-doped　tungsten　powders.　A　lifetime　of　over　10000　h　at　950-degrees　C-b　2-A/cm(2)　dc　loading　in　a　test　diode　has　been　achieved.　Periodic　high　current-density　pulse　testing　was　also　carried　out　during　the　test.　The　performance　for　both　the　dc　and　pulsed　current　densities　remained　stable.　When　tested　at　Stanford　Linear　Accelerator　Center　in　a　cathode　life　test　vehicle　with　a　Pierce　gun　configuration,　the　cathode　operated　for　500　h　at　1170　degrees　C-b,　with　a　pulsed　loading　of　100　A/cm(2)　and　with　less　than　5%　degradation　in　current　density.　The　outstanding　performance　of　these　cathodes　is　attributed　to　a　surface　multilayer　of　Ba-Sc-O　of　about　100-nm　thickness　that　uniformly　covers　the　W　grains　with　nanometer-size　particles　distributed　on　the　growth　steps.　The　layer　is　formed　after　proper　activation　by　diffusion　of　free　or　ionic　Sc　together　with　Ba　and　0　from　the　interior　of　the　cathode　to　its　surface.　This　highly　mobile,　free,　or　ionic　Sc　is　liberated　from　constituents　produced　during　impregnation　and　activation　by　reactions　between　the　matrix　materials　and　impregnants.