In developing countries, medicines may account for 30–40% of health expenditure. Many of these payments are made by individuals purchasing medicines for selfmedication and only rarely on prescription. Understanding how and why consumers make the choices they do is the critical fi rst step to intervening to ensure that these precious resources are spent as safely and productively as possible. This manual is a successor to the 1992 WHO publication, How to Investigate Drug Use in Communities, a small but important book that has been reprinted eight times. A year later came How to Investigate Drug Use in Health Facilities. Since then numerous courses have been held and many studies undertaken, with valuable experience gained in understanding the use of medicines in health facilities and communities. This manual’s authors have been leaders in the movement to better understand and improve medicines use in the community