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On 25 September 2009, the Appeals Chamber of the International Criminal Court (ICC) issued a seminal decision on the subject of complementarity in the case Prosecutor v. Germain Katanga. The outcome of the Chamber's decision is that, even if a state has initiated an investigation or prosecution against an individual, the ICC may prosecute that individual for the same crimes or even a more selective range of crimes, so long as the state is willing to close the ongoing investigation or prosecution at the request of the ICC Prosecutor. While this decision is defensible under the language of the Rome Statute, this article concludes that, absent vigilance on the part of the ICC's Office of the Prosecutor, the decision could produce consequences inconsistent with the principle that the ICC is intended to act as a court of last resort. These potential consequences, in turn, suggest that the prosecutor's policy of 'positive complementarity'- that is, encouraging genuine national proceedings whenever possible - should be at the core of its case selection strategy. At the same time, in those instances when the ICC prosecution determines that, despite activity by a national system with respect to a particular case, it is appropriate for the ICC to take over the case, the prosecution should clearly and publicly explain the factors that led to its decision.