After a respite from mass-casualty terrorism for more than a decade, thus far in 2017 the United Kingdom has suffered three such attacks and a higher tempo of jihadi terrorist plotting than ever before. Absent from the threat picture so far are any Paris-style plots in which the Islamic State has dispatched operatives to launch attacks in the United Kingdom. At this early stage of the investigations, it appears that the Westminster attacker had no contact with the Islamic State and that the Manchester and London Bridge attackers were at most loosely connected to the group. The current threat environment is mostly made up of individuals and smaller scattered cells planning lower-tech attacks with very short planning and operational cycles—sometimes remotely guided by the Islamic State—rather than cells trained and dispatched by the group. But this could change as more British Islamic State recruits return home. With over 20,000 British nationals and residents subject to counterterrorism investigations since 9/11, a growing number of ‘frustrated travelers,’ and a complex and unpredictable set of threats, the United Kingdom faces an unprecedented security challenge.