This article is intended as a tutorial review on the use of single-collector and multi-collector ICPMS for isotope ratio determination. The monitoring and quantification of both induced and natural differences in the isotopic composition of target elements is covered. The capabilities of various types of ICPMS instruments for isotope ratio measurements are addressed and issues, such as the occurrence of mass discrimination and detector dead time effects and appropriate ways of correcting for the biases they give rise to are discussed. Applications relying on induced changes include elemental assay via isotope dilution, tracer experiments with stable isotopes, aiming at a more profound insight into physical processes or (bio)chemical reactions, and nuclear applications. Attention is also paid to the origin of natural variations in the isotopic composition, with focus onto the mechanisms behind the isotopic variation for those elements for which isotopic analysis can be realized using ICP-mass spectrometry, i.e. the occurrence of radiogenic nuclides formed as a result of the decay of naturally occurring and long-lived radionuclides and mass fractionation as a result of thermodynamic and kinetic isotope fractionation effects. Geochronological dating via theRb-Sr, U, Th-Pb, and Pb-Pb methods is briefly explained and also the use of Sr and Pb isotopic analysis for provenance determination studies is covered. Subsequently, applications based on isotopic analysis of elements showing a much narrower range of variation as a result of isotope fractionation are described. Next to provenance studies, such applications include the use of isotope ratios in geochemical, environmental and biomedical studies. Although it is not the intention to comprehensively review the literature, several examples of published applications are used to illustrate the capabilities of both single-collector and multi-collector ICPMS in this context. Thereby, attention is devoted both to widely accepted applications and to more 'exotic' applications, aiming at an extension of the application range.