The subject of this review is the secondary ion mass spectrometry (SIMS) analysis of ultrathin or delta layers of impurity in a semiconductor matrix and their use in establishing the limitations of SIMS depth profiling, exploring the fundamental processes occurring during analysis, and enhancing the quantification of SIMS data. Methods for extracting accurate information for the grower (concerning the material) and the analyst (concerning the SIMS instrument) are described. It is demonstrated that sets of SIMS profiles obtained over a range of analytical conditions are desirable if accurate information is required. In this context, the observation of dopant interaction occurring in codoped samplesduringSIMS analysis is reported for the first time. It is shown that quite large discrepancies exist between different measurements of decay length and associated parameters for the same impurity/matrix combination. These need to be explained before attempting to relate delta profile shape to primary ion beam induced mass transport mechanisms. The concept of the delta profile as a response function and the use of deconvolution as a complete quantification method are discussed. The use of delta profiles in setting up models of the ion–solid interaction such as IMPETUS is illustrated.