The present state of the art of secondary ion mass spectrometry (SIMS), applied to the in-depth analysis of impurity concentration profiles, is reviewed critically. It is shown that SIMS has reached a level of perfection which is unparalleled by other analytical techniques. There are, however, several effects which may cause deviations of the measured profile from the original dopant distribution. These detrimental effects are due to interaction of primary ions with the residual gas, adsorption and incorporation of residual gases, sputtering yield variations due to the accumulation of probe atoms in the sample, mass interference between molecular ions and the atomic species under study and, last but not least, beam-induced relocation of dopant atoms (“atomic mixing”). Methods for minimizing the respective disturbing effect are discussed.