As the clearance rate for homicides remains below 70%, the United States is facing a growing number of homicide cold cases. The dimensions of this crisis are stark: An estimated 250,000 unresolved homicides exist in the nation today. For law enforcement agencies that are implementing strategies to resolve cold cases, it is also important to consider that offenders in these cases are often responsible for multiple crimes. This NIJ Journal article focuses on cold cases involving serial homicides. It outlines NIJ’s role in supporting efforts to solve these cases — for example, through the Solving Cold Cases with DNA Program and the National Missing and Unidentified Persons System — and profiles a number of serial killers who were caught or identified with help from NIJ programs. The article also describes how newly available techniques and tools can help catch serial offenders early, potentially saving lives and preventing future crime.