Paedophile hunting. Phishing. Cyber addiction. Revenge porn. Disinformation. These are just a few examples of harmful and immoral behaviour online. Certain properties of the internet – the virality of online messages, the (perceived) anonymity of internet users, and the immediacy with which a video can be viewed worldwide – facilitate these kinds of behavioural phenomena. They can be enormously harmful for both individuals and society. But what kind of behaviour are we talking about? How prevalent is it? And what can the Dutch government do about it? At the request of the Research and Documentation Centre for the Dutch Ministry of Justice and Security (WODC), the Rathenau Instituut has studied harmful and immoral behaviour online.