As part of its term of reference to examine the effectiveness of the criminal justice system in securing the conviction of the guilty and the acquittal of the innocent, the Runciman Royal Commission will consider whether changes are needed in the conduct and supervision of police investigations. This paper argues that the fundamental issue to be addressed is the integrity of evidence, a problem which has been of concern to every Royal Commission into the police this century yet which remains outstanding. It argues that it is now both necessary and politically acceptable for ACPO to use the commission as a catalyst to increase its leadership and policy setting role to address the problem nationally from a central perspective. It examines integrity in relation to evidence gathering and attempts to define its importance, to assess how it is lost and to recommend how it can be restored. It suggests that whilst systems have value in this respect they do not address the underlying behaviour or the culture from which malpractice springs and that, whatever the commission recommends, it is the more fundamental issue of culture which will have to be addressed by internal central direction. Rather than wait for changes to be imposed from outside the service it recommends that ACPO should seize the initiative, carefully research all issues, design and introduce new internal systems to improve procedures and then introduce and promulgate an applied code of ethics to enhance the professionalism and culture of our organization. It envisages that ACPO might wish to invoke a joint association approach, similar to that involved in the preparation of the Operational Policing review, to do so. Having put its own house in order, it suggests the service should then submit details of its internal response to the commission together with its carefully researched recommendations for other ‘external’ changes which would improve the whole criminal justice system. It contends the price of not responding in such a positive manner may be the imposition of working practices less effective than those the service might design - perhaps cataclysmic change and a lost opportunity to radically improve the judicial system. Cataclysm or catalyst? The choice it suggests is ours.