Irrational use of medicines is one of the most serious global public health problems we face today. Improving use requires regular monitoring and application of effective interventions, both of which are often not done, particularly in many lower income countries. To address this issue, WHO created a database of medicines use in in primary care settings in developing and transitional countries. Quantitative information has been extracted from 679 studies conducted in 97 countries and on 386 interventions (from 313 studies). The medicines use database has allowed the first systematic quantitative review of studies measuring medicines use in developing and transitional countries and the results are now presented. How serious a problem is irrational use of medicines? What interventions are effective?