Online Social Networks and Media have revolutionized society, and are now a key part of how most people work, live, socialize, find information and entertain themselves. But whilst they have generated huge benefits, leading to unprecedented connectivity across the globe, online social networks have also enabled the spread of hazardous and dangerous behaviours. Such ‘online harms’ are now a pressing concern of policymakers, regulators and big tech companies. Building deep knowledge about the scope, nature, prevalence, origins and dynamics of online harms is crucial for ensuring we can clean up online spaces. This, in turn, requires innovation and advances in methods, data, theory and research design — and developing multi-domain and multi-disciplinary approaches. In particular, there is a real need for methodological research that develops high-quality methods for detecting online harms in a robust, fair and explainable way.