The aims of the Editorial are to summarise what we know for certain from clinical trials of Intensive Case Management, and to highlight lessons for clinicians and researchers. I will upon two systematic reviews of trials of Intensive Case Management versus standard care or low intensity case management. Both incorporated a meta-regression which examined the effect of fidelity to the Assertive Community Treatment model on outcome. The effectiveness of Intensive Case Management was limited to improving patient satisfaction and reducing attrition. Intensive Case Management teams organised according to the Assertive Community Treatment model offered the additional benefit of reducing days in hospital, but only when the team's clients had been high users of hospital care over the previous 12 months. Four important lessons can be drawn: a) Changes to the process of care tend to affect process variables, not outcome variables. b) Complex interventions must be defined meticulously in clear terminology. c) Researchers must demonstrate that complex interventions have been properly implemented in clinical trials. d) It is important to remember that in a clinical trial a successful outcome is determined as much by the control group as by the intervention.