The aim of this paper is to present some recent advances in the diagnostic of solar prominences, with an emphasis put on the results obtained with SOHO through its six years of observations. We show the progress made since the Hvar Reference Model in the determination of basic thermodynamic parameters, especially in the Prominence-Corona Transtion Region (PCTR). We address the important contribution of ``prominence seismology" for determining e.g. density and magnetic field. We summarize the most recent comparisons between non-LTE modelling predictions and spectra of optically thick lines. Finally, we mention the most recent magnetic measurements which seem to challenge the historic Hanle measurements of Leroy (1988) as summarized in Bommier et al.  (1994).