We analyzed spin models with anisotropy, both from the classical and quantum point of view. In the classical case we proved the existence of a disconnected phase space below a critical energy threshold: the Topological Non--connectivity Threshold (TNT). In  particular we proved that while the anisotropy of the coupling is the only necessary ingredient to induce the presence of the TNT, in the large N limit, only for long range interacting systems the disconnected energy portion determined by the TNT remains finite w.r.t. the total energy range of the system. We also analyzed the consequences of the TNT with respect to ferromagnetic/paramagnetic behavior. On the quantum side we analyzed the consequences of the TNT with respect to Macroscopic Quantum phenomena.  Possible connections with recent experiments are also discussed.