This chapter is dedicated to introduce information we can derive from optically thin emission of prominences plasma. This emission comes mostly from the interface region with the corona, the prominence-corona transition region, and it is observable in the UV-EUV wavebands. After a general introduction to the formation of the optically thin emission, we present the diagnostics methods which are used to infer the thermal properties of the emitting plasma under isothermal and multi-thermal hypothesis. We then describe a diagnostics technique to infer the electron density. For each method presented we give advantages and limitations, together with the main results. We then discuss the diagnostics at small, unresolved scales introducing the filling factor and conclude with some final remarks.