Although this pamphlet is intended as background material for teachers in English primary schools changing to the System International d'Unites (SI units), the form of the metric system being adopted by the United Kingdom, the educational implications of the change and the lists of apparatus suitable for use with children up to 14 years of age are sufficiently general for use in other countries introducing the metric system of weights and measures. Some of the teaching ideas are applicable to introducing metric units for science instruction. (AL)