In the School of Mathematical Sciences at Queen Mary in the University of London we have been running a module that teaches the students to write good mathematical English. The module is for second-year undergraduates and has been running for three years. It is based on logic, but the logic—though mathematically precise—is informal and doesn’t use logical symbols. Some theory of definitions is taught in order to give a structure for mathematical descriptions, and some natural deduction rules form a basis for writing mathematical arguments. Alongside this logical material, the students have weekly exercises that involve writing informal explanations of simple mathematical ideas for non-mathematicians.