Interpretation of observations by inversion

The most recent developments in inversion techniques of the radiative transfer equation are critically reviewed and some of their findings are summarized to illustrating their achievements. Two significantly different approaches are currently being used that deserve consideration, each characterized by whether or not the model solar atmospheres are changed iteratively by the algorithm. The comparison between the two may help in finding future inversion techniques that can solve many challenging problems of solar physics that still need to be properly settled. These problems themselves suggest strategies that look more suitable than others.