Pittsburgh. San Bernardino. Charlottesville. Charleston. These tragic events, now seared into the history of the United States, represent the evolution of terrorism in the United States. Since the 9/11 terror attacks, the terrorism threat landscape has shifted significantly and continues to evolve. The threat posed to the United States has expanded from sophisticated, externally directed plots to attacks conducted by self-radicalized lone actors who mobilize to violence based on international and domestic violent ideologies. This divergence from traditional to nontraditional acts of terrorism creates challenges for the U.S. law enforcement community as it tries to identify and disrupt both international and domestic terrorist plots, as well as hold accountable those who carry out these attacks. As the threat evolves, law enforcement’s response must also evolve to combat the actors of today, including considering how federal, state, and local agencies can work together with the communities they serve to mitigate the threat and ensure national security.