This paper is an analysis of the uses of data generated by the federal program of tracing the origins of guns used in crime and confiscated by the police. The authors begin with details on the tracing process and new tabulations based on 1999 data for how guns are successfully traced, and the reasons for failure when they are not. Section C provides a brief history of the development of comprehensive tracing as a component of the supply-side strategy conducted by the Clinton administration. Section D discusses the statistical relationship of the trace "sample" to the relevant "population" of guns in the hands of criminals. Section E provides an analysis of what the trace data tell us about trafficking patterns: that newer guns are over-represented in crime even though criminal users are rarely among the first purchasers, and that the percentage of crime guns imported from out of state tends to be closely linked to the stringency of local controls. Taken together, these findings suggest that licensed dealers are playing a significant role in "supplying the suppliers" of guns to criminals, and that firearms trafficking may be one of the important channels by which guns reach criminals, especially in the tight-control states. Section F then reviews the case for using trace data to guide ATF's enforcement efforts against specific dealers and traffickers, and the limitations of this approach. Section G illustrates the use of these data in the evaluation of gun-control laws. The final section offers conclusions concerning the promise and pitfalls of wholesale tracing.