A wide range of methods is now available for assessing the nature and characteristics of drug injecting populations, and for evaluating the effectiveness of interventions developed to reduce injecting related harms. The public health surveillance tasks in relation to injecting drug use populations and associated health problems are the same, in principle, as for the surveillance of other health problems. These are: to describe the patterns of the condition, the nature of the problem and the environment (context) in which it occurs; to determine the scale of interventions needed and estimated coverage required, to forecast future health care needs; to mobilise resources and target prevention; and to evaluate interventions. Countries vary in their existing levels of information as well as resources for surveillance systems, research and evaluation. We propose three levels of assessment: basic assessment, which is suitable in situations of low awareness and information, routine surveillance, and enhanced surveillance, which requires more complex research and/or analyses of data collected from routine surveillance. © 2005 Elsevier B.V. All rights reserved.