Beginning with some introductory remarks about aims and scope, historical background and present state of the art, a brief survey of the technique of sputter depth profiling is given. The fundamental principles of ion/solid interactions during sputtering and the resulting changes in surface composition and surface topography as well as their influence on the conversion of sputtering time to sputtered depth are illustrated by some examples. Together with the specific analysis method employed, such as secondary ion mass spectrometry or Auger electron spectroscopy, these effects determine the shape of the depth profile of interfaces and of thin layers. Recent developments in evaluation and quantification of depth profiles are reviewed with emphasis on the role of the depth resolution function in profile reconstruction, including experimental determination and theoretical modelling of the depth resolution function by the three fundamental parameters: atomic mixing, surface roughness and information depth (MRI model). A summary of optimized experimental conditions for high-resolution depth profiles highlights the use of sample rotation and of low-energy primary ions. Practical application of depth profiling at interfaces is further elucidated by typical examples in the fields of surface and grain boundary segregation, corrosion, oxidation and interdiffusion controlled reactions at thin-film interfaces in electronic materials. An outlook on future trends focuses on theoretical profile simulation methods and on instrumental developments.