IEC 60268-16:2020 defines the STI model, test signals, measurement and prediction methods. The objective of this document is to provide a comprehensive manual for all types of users of the STI model in the fields of audio, communications and acoustics. This document does not provide STI criteria for certification of transmission channels (e.g. criteria for a voice-alarm system), but some typical application values are provided in Annex G. Every measurement method has limitations, and the reader is referred to clauses relating to limitations such as speech privacy, echo and systems using digital voice compression (vocoders). This document does not cover the case of fluctuating noise on the STI, although some general comments on dealing with this complex issue are provided in 7.13 and 8.9.3. IEC 60268-16:2020 cancels and replaces the fourth edition published in 2011. This edition constitutes a technical revision. This edition includes the following significant technical changes with respect to the previous edition: a) the spectrum of the male speech test signal has been changed, with significant reductions in the 125 Hz and 250 Hz bands being implemented; b) some corrections to formulae have been made; c) additional information has been included on prediction and measurement procedures; d) spectrum and weighting factors for female speech have been removed; e) verification information for STI measurement devices added; f) the relationships between STI and number of other speech intelligibility measures have been updated in Annex E; g) greater information is given in Annex M about adjustments to the measured STI results to simulate effects of alternative ambient noise and speech levels.