Radiative transfer is underlying physical phenomenon in many astrophysical problems and among the most difficult phenomena to deal with. The main difficulty arises from the non-local and, in general, non-linear coupling of the radiation field and the state of the gas. A variety of numerical methods have been developed so far to solve the NLTE radiative transfer in spectral lines (NLTE line formation problem). An overview of selected methods will be given in the talk. Special emphasize is put on two extremely fast convergent methods: Iteration Factors Method (IFM) and Forth-and-Back Implicit Lambda Iteration (FBILI) that use the so-called iteration factors in an explicit and implicit way, respectively. Although these methods are developed to solve the general non-linear multi-level problem, their basic ideas and properties are demonstrated on the well-known test-problem of the two-level atom line formation under the assumption of complete redistribution.