Each year, gun homicides kill over ten thousand people in the United States. While most guns are initially purchased legally, many enter the underground market and end up in the hands of dangerous offenders, such as gang members and convicted felons. This issue of RSF, edited by public policy scholars Philip J. Cook and Harold A. Pollack, brings together eight articles exploring the markets for guns, both legal and illegal. The contributors draw from new datasets and interviews with inmates to examine how offenders obtain guns. By turning a spotlight on these little-understood supply chains, these chapters provide guidance for stemming the flow of guns to the underground market.
 
Cook and Pollack initiate the discussion with a comprehensive introduction that includes an original framework for understanding the legality of transactions that arm dangerous offenders.  Several contributors review trends in gun ownership across the nation and investigate how guns enter the underground market. Deborah Azrael and coauthors conduct a comprehensive survey of gun owners and find 270 million guns in private circulation. They estimate that 70 million firearms changed hands during a recent five-year period.  Most of those transactions were sales by licensed dealers; Garen Wintemute  reports on a survey of dealers that assesses the wide disparities among them as sources of guns to offenders.  Charles Wellford and coauthors use trace data and prison interviews to study how criminals obtain their guns in three jurisdictions that differ widely with respect to gun regulations and culture. 
 
Other contributors explore the effects of gun regulations and legislation on illegal supply chains. Daniel Webster and coauthors study the effects of Maryland’s 2013 Firearm Safety Act, which required all handgun purchasers to obtain a license. They find that 41 percent of surveyed parolees reported that it was more difficult to obtain a handgun after the law passed due to increased cost, lack of trusted sources, or people being less willing to buy handguns on their behalf. Analyzing over three decades of data on handguns recovered in Boston, Anthony Braga shows that fewer guns are illegally obtained from states that adopt legislation restricting buyers to one gun per month.  George Tita and coauthors  report the results of an inmate survey in Los Angeles in which they explored respondents’ knowledge of gun laws.  These findings suggest that laws aimed at legal gun transactions may also decrease the entry of guns into the underground market. 
 
The rate of gun violence in the U.S. surpasses that of any other advanced nation. This issue of RSF offers new empirical research on the underground market that arms dangerous criminals and provides a rich foundation for policies designed to curb gun violence.