Prices are an essential element of the illicit drug market, from both a supply and demand perspective. They are an indicator of drug availability and are an important tool for understanding the workings of drug supply mechanisms. They may also be used to help target law enforcement resources and assist criminal justice agencies with decisions related to prosecution and sentencing (e.g. assets recovery). The EMCDDA has been collecting information on retail drug prices via its Reitox focal points for the last 15 years. Although considerable progress has been made in this area, there is still variation in data collection in Europe. To address this issue, the EMCDDA has released these Guidelines for collecting data on retail drug prices in Europe: issues and challenges in its Manuals series. The Manual raises awareness on specific issues related to collecting data on retail drug prices and offers practical suggestions. It addresses difficulties in obtaining a definition of 'retail' prices, and discusses the challenge of obtaining a representative sample of illicit, and therefore hidden, transactions. Suggestions are made for a minimum set of variables to be recorded for each observation. Targeted at all those involved in data collection on drug prices in Europe, the Manual reviews the main data-collection methods, current national practices and data-management issues.