Isotope ratio measurements are required for a variety of applications: accurate quantification of element concentrations using isotope dilution; tracer experiments using enriched isotope spikes in environmental, biological or medical sciences; determination of isotope abundances of radiogenic elements and their products in the nuclear industry, in environmental studies (e.g. for tracing pollution sources) and for geological age determination (dating); investigations of isotope variations of stable isotopes in nature. During the last two decades, inductively coupled plasma mass spectrometry (ICP-MS) has been increasingly used for such applications, becoming a significant complement to thermal ionization mass spectrometry (TIMS). The short course will be focused on accurate and precise isotope ratio measurements using double-focusing sector field ICP-MS (ICP-SFMS). Equipped with a single ion collector, this technique provides short-term precision in the 0.02%-0.1% RSD range at best. Developed a decade ago, multi-collector ICP-MS (MC-ICP-MS) allows significant improvements in precision down to a few ppm. However, high precision does not always guarantee high accuracy. In order to obtain true isotope ratios, measured ratios have to be corrected for all possible spectral interferences on analyte masses; for detector ‘dead time' (for detectors operated in ion-counting mode) and ‘sag' of electronic circuits; for mass bias; for possible blank contribution. Different approaches for performing such corrections will be discussed. The accuracy of isotope ratios measured by single collector ICP-SFMS in a variety of samples will be illustrated using results from several international performance evaluation exercises.