The article presents experimental and theoretical investigations on the dynamics of saw-teeth and neoclassical tearing modes in tokamak plasmas. The main findings of these investigations are: (1) The experimental validation, on the basis of FTU and TCV data, of a model for the prediction of the sawtooth period and amplitude; (2) the importance of diamagnetic effects in determining the threshold for sawtooth crashes and the rotation of non-linear tearing modes; (3) a clarification of collisionless reconnection processes, in particular the role of phase mixing in association with magnetic island growth and saturation in dissipationless regimes. The interplay between ECRH and macroscopic island dynamics is also discussed.