Homicide clearance rates, at 61 percent nationwide, are disappointing, and clearance rates for nonfatal gun shootings are even lower. Phillip Cook, Professor Emeritus of Public Policy and Economics at Duke University, offers explanations for relatively low clearance rates, including changes in arrest requirements and the mix of cases being investigated. For example, as drug and gang cases have proliferated, clearance rates have declined. On the positive side, Cook discusses promising efforts by Denver, Indianapolis, and other cities to boost clearance rates.