One of the key remaining challenges for HPM (> 100 MW, 100 A/cm2 and negligible evidence of cathode plasma. Experimental I-V characteristics of monolithic-metal cathodes suggest anomalously high local surface electric field enhancements (> 100) combined with small fractions ( 0.01) of the cathode surface responsible for the emission. These collective observations raise fundamental questions about the mechanisms and limitations of the electron emission. This talk summarizes the findings on novel cathode performance, emission mechanisms, the role of surface nano-and micro-scale properties, and emission constraints posed bv the space-charge-limited Fowler Nordheim condition.