This article addresses lessons learned from selected mass casualty terror attacks in Western Europe (Nice, Paris, Brussels and Oslo) and the United States (Orlando, San Bernardino and Boston). In particular, the piece reviews these terror incidents, and addresses their consequences on emergency management and medical response planning. The March 2017 terror attacks in London resulted in four deaths and more than 50 injuries, underscoring the continued specter of these incidents and the crucial roles first responders and medical professionals play in addressing them.