SIMS depth profiling is now expected to extract accurate quantitative data from the top 10 nm of a wafer under circumstances where a significant part of the measurement may be in the top 3 nm, and where the total impurity level in the material may be above 1%. The reality is that, however reproducible the data, accurate profiles can only in general be obtained for impurity levels below 1% and in the depth range 3-10 nm by very careful selection of the analytical conditions. For high dose, ultra-shallow, implants using molecular ions such as BF@sub 2@ (where the total impurity level may be as high as 30%) or for thin (1-5 nm thick) dielectric layers the inherent nonlinearity of both sputtering and ion emission, combined with the fact that the region of interest overlaps with the transient region leads to very strong matrix effects for which no correction procedure has yet been devised (indeed, depending on the level of non-linearity, no correction procedure may be possible). Given that there is an insatiable demand for profiles from the problem region outlined above, how can one obtain accurate profiles, or at least establish the level of error in, say, the dose, junction depth, or internal profile from a 2 nm thick oxynitride layer? The general answer is to examine profiles obtained under different analytical conditions and from different analytical techniques to see if the data converge on a dose or even a shape, to measure changes in erosion rate in the transient region using techniques other than SIMS (e.g. MEIS), and to compare directly profiled data with data where the transient has been removed from the problem by capping the wafer. In addition, measurements should be made under the most linear conditions possible so that reasonable correction procedures can be devised. Of course, suitable reference materials are required, in order to establish such conditions. Techniques for accurate profiling of the top few nm of a wafer still require research and further development of the tools before claims to a reproducible dose and junction depth can be translated into a known accuracy in profile shape, dose and junction depth.