This special issue explores the long-term impacts of the July 22 attacks in Norway in 2011, carried out by an extreme right terrorist lone actor. The Introduction article will first describe the car bomb attack on the Government District in Oslo and the subsequent mass shooting of young participants at the Labour Party’ youth organisation at the Utøya Island. Next, it will discuss dimensions of impact at three levels: the individual, the national/societal, and the global levels. Finally, the article asks why some terrorist attacks have stronger and more lasting impacts than others, comparing the July 22 attacks with some other large-scale terrorist atrocities (like the 9/11 and 7/7 attacks in the US and UK) but also some small-scale but nevertheless significant attacks. Three factors stand out: Severity (in terms of casualties and other harm), innovation (the terrorists did something different that became extra shocking and newsworthy), and responses (from governments, the public or potential supporters).