Irrational prescribing is a “disease” which is difficult to treat. Prevention is possible however. For this reason the WHO Department of Essential Drugs and Medicines Policy aims to improve the teaching of pharmacotherapy to medical students. The Teacher’s Guide to Good Prescribing is a companion volume to the Guide to Good Prescribing. Its target audience is university teachers who wish to use the Guide for teaching undergraduate medical students. Its main objectives are: to explain the educational approach underlying the Guide; to explain how to teach pharmacotherapy with the Guide; to give practical advice on how to assess the students, the teachers and the course; and to assist in mobilizing support for problem-based pharmacotherapy teaching.