Edward Snowden speaks at the recent 2014 Cato Institute Surveillance Conference. Over the past year, the disclosures spurred by former National Security Agency contractor Edward Snowden have drawn public attention to the stunning surveillance capabilities of the American intelligence community, and the unprecedented volume of data they collect from hundreds of millions of people around the world. But the growth of government surveillance is by no means restricted to spies: Even ordinary law enforcement agencies increasingly employ sophisticated tracking technologies, from face recognition software to “Stingray” devices that can locate suspects by sniffing out their cellular phone signals. Are these tools a vital weapon against criminals and terrorists — or a threat to privacy and freedom? How should these tracking technologies be regulated by the Fourth Amendment and federal law? Can we reconcile the secrecy that spying demands with the transparency that democratic accountability requires?