This chapter examines the state of the literature–including scientific research, legal scholarship, and policy reports–on online hate speech. In particular it explores ongoing debates and limitations in current approaches to defining and detecting online hate speech; provides an overview of what social media data and surveys can tell us about the producers, targets, and overall prevalence of harmful language; reviews empirical evidence of the offline consequences of online hate speech; and offers quantitative insights into what interventions might be most effective in combating harmful rhetoric online.