The trafficking of drugs and, in particular, the use of human carriers (often called ‘drug mules’) remains a relatively hidden phenomenon, on which limited information is available. This latest EMCDDA Thematic paper explores whether a common definition of ‘drug mules’ can be developed in the European context and assesses the implications of this for data gathering and future research. The paper has its roots in a questionnaire launched by the EMCDDA in 2010 to test a proposed conceptual framework of drug couriers. Taking part in the survey were experienced professionals, academics and practitioners from a variety of countries, legal traditions and law-enforcement practices. Drug mules are reported to be transporting all of the major illicit drug types through Europe and making full use of all European transport routes. This paper draws a number of conclusions and recommendations for further investigation into drug markets and related responses.