Sputter depth profiling is a direct method to obtain the in-depth distribution of composition in thin films, at surfaces and at interfaces. It was developed in conjunction with the main methods of surface and interface analysis (AES, XPS, and SIMS). During the past decades, progress in depth profiling is characterized by the aim to obtain optimally resolved depth profiles. This is mainly achieved by experimentally identifying the decisive parameters for distortions of the original in-depth distribution caused by sputtering and analysis. The physical origins of profile blurring are predicted by elaborate particle transport theories based on Sigmund's theory of sputtering. A simplified, user-friendly model presentation of the result for depth profiling is its representation by convolution of the original in-depth distribution with an appropriate depth resolution function (or response function). Great progress has been achieved by the development of the so-called MRI model, based on the three physically well-defined parameters atomic mixing, roughness and information depth, which describe the depth resolution function. The latter can be presented as a user-friendly analytical expression for delta layers as well as for layers with a given thickness. Numerical solutions are necessary to include preferential sputtering and other depth or concentration-dependent parameters. Whereas the instrumental development in AES is fairly stable, great progress is going on in XPS and particularly in SIMS and related techniques, enhanced by growing interest in three-dimensional analysis of organic materials. At the horizon, there are more sophisticated models for quantification of transient states in shallow profiles and of sputter depth profiles generated by cluster ion bombardment of organics. Copyright © 2014 John Wiley & Sons, Ltd.