Critical information about prominence physical conditions can be obtained only by analyzing prominence spectra or high-resolution monochromatic images. Solar prominences are low-density structures and, therefore, the non-equilibrium (NLTE) spectroscopy tools have to be used. We describe the basics of the NLTE radiative-transfer theory applied to typical prominence conditions and demonstrate why the NLTE approach is necessary. Starting from the simplest 1D slab models, we gradually move to more complex problems which include 2D transfer, importance of partial redistribution in the formation of resonance lines and details of multilevel problems including the most up-to-date numerical methods for constructing NLTE models. We also mention some recent achievements in prominence modeling (e.g. magnetic dips, radiative equilibrium models, oscillations), although detailed description of such results is the subject of other chapters.