The advent of very large telescopes equipped with powerful spectrographs now routinely collect spectra from the near ultraviolet to the infrared wavelengths at an unprecedented quality, revealing spectral details that were not detected. The analysis of high resolution spectra needs to include modelling techniques which include NLTE physics, certainly for specific lines. To establish a better familiarity with NLTE spectrum analysis, a workshop was conducted in Nice, France at the beginning of August 2007. The workshop provided a gentle and progressive introduction to semi-consistent NLTE modelling of stellar atmospheres (i.e. based on an underlying LTE model atmosphere) for researchers, students and post-docs normally familiar with using LTE but concerned by possible NLTE effects in the lines they study. Morning sessions were comprised of lectures that addressed radiative transfer in stellar atmospheres, the LTE and NLTE solutions to the equation of radioative transfer, statistical equilibrium for trace elements, atomic data and the construction of model atoms, NLTE analysis of observed spectra, background opacities, and mechanisms of departures from LTE in various types of stars. The proceddings of the workshop reflect all topics presented. Each chapter has been written and edited to be a comprehensive introduction to the subject of non-LTE analysis for abundance analysis of trace elements in stellar atmospheres. The text will therefore be of interest to a much wider community than the participants of the workshop, and it may serve as a textbook on the subject.