"The Language of Mathematics in Science" is an ASE/Nuffield project aimed at supporting teachers of 11-16 science in the use of mathematical ideas in the science curriculum. Two publications have been produced. This article focuses on the first of these, "The Language of Mathematics in Science: A Guide for Teachers of 11-16 Science," which provides an overview of key ideas and approaches in this area, along with a glossary of relevant terms. A selection of teacher discussion activities are introduced to illustrate some of the general themes that run through the publication. One key theme is that, while there are important mathematical techniques to be learned, the way that mathematics is used in science is a matter of judgement and not of applying hard-and-fast rules.