OBJECTIVE To provide an overview of and critically appraise classifications of drug-related problems (DRPs) for use during the pharmaceutical care process and research in pharmacy. DATA SOURCES A literature search was conducted using MEDLINE and Yahoo (January 2003) and manually. The search terms included DRP, drug-related problem, drug-therapy problem, and medicine-related problem. STUDY SELECTION AND DATA EXTRACTION English- and German-language articles on pharmaceutical care and DRPs were reviewed. DATA SYNTHESIS Most classifications of DRPs were identified through searching publications on pharmaceutical care and DRPs. Fourteen classifications with different focuses were found. Some classifications were hierarchical, categorized into main groups and subgroups. Various terminologies and definitions for DRPs were revealed, as well as guidelines for an optimal DRP classification. Classifications were assessed according to a clear definition, published validation method, and results reflecting process and outcomes, usability in pharmaceutical care practice, and a hierarchical structure with main groups and subgroups. CONCLUSIONS Finding DRP classifications by computerized search of the biomedical literature with the help of PubMed proved to be difficult. No classification could be found that met all of our criteria for an optimal system. Few classifications have been validated. Three have been tested as to their usability in practice and internal consistency. The Pharmaceutical Care Network Europe system Version 4 comes closest to the defined requirements.