This surveillance activity is designed to monitor trends so that the effectiveness of interventions can be evaluated. - a study, funded by the Effective Interventions Unit, to evaluate the effectiveness of the raising of the limits on the numbers of needles and syringes made available to injecting drug users; a report on the findings of this study will be published in 2005. - a study, funded by the Chief Scientist Office, to evaluate the impact of methadone maintenance on high risk behaviours among injecting drug users, which was completed last year.