The first IUPAC Manual of Symbols and Terminology for Physicochemical Quantities and Units was published in 1969 with the objective of 'securing clarity and precision, and wider agreement in the use of symbols, by chemists in different countries, among physicists, chemists and engineers, and by editors of scientific journals'. Subsequent revisions have taken account of many developments in the field and were also substantially expanded and improved in presentation in several new editions of what is now widely known as the ‘Green Book of IUPAC’. This abridged version of the forthcoming 4th edition reflects the experience of the contributors and users of the previous editions. The book has been systematically brought up to date and provides a compilation of generally used terms and symbols with brief, understandable definitions and explanations. Tables of important fundamental constants and conversion factors are included. In this abridged guide, the more specialized and complex material has been omitted, retaining, however, the essence of the Green Book. It is particularly intended to be suitable for students and teachers but it should also be useful for scientists, science publishers and organizations working across a multitude of disciplines requiring internationally approved terminology in the area of Physical Chemistry. It now includes the most up to date definitions and constants in agreement with the ‘new SI’ as established by agreement on the International System of Units in Paris in 2019. It should find the widest possible acceptance and use for best practice in science and technology.