Since the 1970s the United States and other nations have conducted regular statistical monitoring of the prevalence and patterns of drug use. Given the importance of such surveys for policymaking, their quality is a critical issue and the biases that may affect their measurements a major concern. An increasing volume of empirical evidence shows that the mode of administration of a survey can strongly influence the validity of respondents' reports. Compared with interviewer-administered questionnaires, self-administered forms appear to elicit more complete reporting of drug use, but the challenges they pose to the literacy skills of respondents may result in measurement biases. Strategies to improve monitoring of trends in drug use include: (1) more frequent use of a survey technology that ensures privacy for respondents but does not require literacy; (2) increased use of time-series of indicators of drug use consequences built from blinded surveys of medical records; and (3) population-based surveys that collect biological specimens. Data from the latter two sources are not subject to the same constellation of biases that affect self-reports of drug use. Time-series of those data can be integrated with self-reports to provide a better understanding of changes over time in the prevalence and patterns of drug use. Notes, references