This report presents the findings of a project that investigated the prevalence, scale and nature of the ideologies and narratives that motivated the attack which left 51 dead and injured a further 50 more during Friday prayers at the Al Noor Mosque and the Linwood Islamic Centre in Christchurch, New Zealand. The report explores the origins of the ‘Great Replacement’ and ‘white genocide’ theories and the dynamics and platforms which have allowed the ideas to spread. Using a combination of quantitative and qualitative analysis across mainstream and alternative social media channels, this report reveals significant mainstreaming of extreme-right narratives across social media platforms and in language and policies promoted by mainstream politicians in Europe and North America. This report provides some recommendations for steps to be taken by technology companies, government and civil society to counter the spread of extremist ideologies.