Massive (perturbative) string states are interesting in many respects (cosmology, black hole Physics, production at accelerators). We review the research done in order to find the perturbative massive superstring states with very long lifetime, the characteristics of their decay, the emission spectrum, both in fully extended space-time and in the case of toroidal compactifications. The main result is the identification of a particular state with very long lifetime decaying only by gravitational emission and with thermal-like spectrum.