6.1 HFQS Summary6.2 HFQS in Low-β Cavities6.3 Deconvolution of RBCS and Rres6.4 Depth of Baking Effect6.5 Role of the Oxide Layer and Role of N-Infusion6.6 SIMS Studies of O, H, and OH Profiles6.7 Hydrogen Presence in HFQS6.8 TEM Studies on Hydrides6.9 Niobium–hydrogen Phase Diagram6.10 H Enrichment at Surface6.11 Q-disease Review6.12 Visualizing Niobium Hydrides6.13 Model for HFQS – Proximity Effect Breakdown of Nano-hydrides6.14 Positron Annihilation Studies of HFQS and Baking Effect6.15 Point Contact Tunneling Studies of HFQS and Baking Effect