Vigilantism – popularly understood as 'taking the law into your own hands without any legal authority' – is an old phenomenon that ranges from lynchings of cow thieves or blacks to anti-crime street patrols and armed militia groups. Vigilantes typically direct their activities against alleged criminals or people claimed to break a moral order, often justified by alleged lack of action by the authorities. Online vigilantism is a modern twist. Many examples are presented in this edited volume. The editors define 'digitally mediated vigilantism to include practices where digital media users are offended by others and retaliate through practices and repertoires that include mobile devices and social platforms'. The case studies analyse how social or digital media are used to shame individuals who (allegedly) have broken legal or social norms. Such activism is often based on moral indignation, using unfavourable exposure to undermine the social standing of the targets, or even cause them to lose their jobs. Some forms of public shaming can easily be considered virtuous, as the doxing of perpetrators of racist hate speech or militant far-right rioters in Charlottesville or those storming the Capitol. Other forms are highly problematic, such as outing sexual minorities in Russia, by claiming they are paedophiles. Even relatively minor forms of ridicule and shaming can culminate in insurmountable harm. For instance, an elderly woman was caught on camera committing a minor theft. When her act and identity was spread on social media, she committed suicide. Although the proclaimed goal of the digital vigilantes is usually to uphold certain moral values, there is a confluence of informal policing with entertainment, and the outcome may be severe and unpredictable. This is a useful and thought-provoking volume.