In tokamaks and stellarators, measurements of electromagnetic fluctuations in the presence of resonant particle drive, including fusion-produced alpha's, reveal the excitation of Alfven eigenmodes (AE), related under certain conditions to a degradation in the fast-particle confinement. The balance between the drive and the background damping is investigated using active diagnostic systems to excite and measure the AE spectrum in terms of frequencies and damping rates. At JET, saddle-coil antennae drive low toroidal mode number (n < 4) AE in the range 30-500 kHz, including toroidal AE, kinetic AE, elliptical AE and global AE. Conditions for weak damping (gamma/omega(damp) < 1%) are identified. Low-n AE appear to be strongly damped (gamma/omega(damp) > 1%) during the creation of the magnetic X-point. In the presence of resonant fast particles, information on the instability drive is obtained: low-n modes are found to be stable in the presence of NBI with upsilon(parallel to)/upsilon(A) < 1. Fast ions generated by ICRH are observed to produce a drive for 4(ICRH) > P-thresh, with 2.5 MW < P-thresh < 5 MW; under these conditions, intrinsically driven TAE and EAE are clearly observed in the magnetic fluctuation spectra, with no measurable effect on the plasma performance.