Measurements of high frequency oscillations in JET, JT-60U, Alcator C-MOD, DIII-D and TFTR plasmas are contributing to a new understanding of fast ion driven instabilities relevant to Advanced Tokamak (AT) regimes. A model based on the transition from a cylindrical-like frequency-chirping mode to the Toroidal Alfvén Eigenmode(TAE) has successfully encompassed many of the characteristics seen in experiments. In a surprising development, the use of internal density fluctuation diagnostics has revealed many more modes than has been detected on edge magnetic probes. A corollary discovery is the observation of modes excited by fast particles traveling well below the Alfvén velocity. These observations open up new opportunities for investigating a "sea of Alfvén Eigenmodes" in present scale experiments, and highlight the need for core fluctuation and fast ion measurements in a future burning plasma experiment.