With the new generation of radiative transfer solvers and spectropolarimetric inversion codes, we are now able to synthesise and invert NLTE spectral lines. This allows us to synthesise observations using realistic MHD simulations or, using observation, extract information about the temperature, velocity, and magnetic field vector at different layers of the solar atmosphere from the photosphere to the chromosphere. Although the currently available codes are reaching a high level of maturity, several aspects are still open to improvement and discussion. In this workshop, we will discuss the advantages and limitations of each code and possible improvements that are planned or need to be addressed.