Online Harassment is one of the most serious problems in social media. To address it requires understanding the forms harassment takes, how it impacts the targets, who harasses, and how technology that stands between users and social media can stop harassers and protect users. The field of Human-Computer Interaction provides a unique set of tools to address this challenge. This book brings together experts in theory, socio-technical systems, network analysis, text analysis, and machine learning to present a broad set of analyses and applications that improve our understanding of the harassment problem and how to address it.  This book tackles the problem of harassment by addressing it in three major domains. First, chapters explore how harassment manifests, including extensive analysis of the Gamer Gate incident, stylistic features of different types of harassment, how gender differences affect misogynistic harassment. Then, we look at the results of harassment, including how it drives people offline and the impacts it has on targets. Finally, we address techniques for mitigating harassment, both through automated detection and filtering and interface options that users control. Together, many branches of HCI come together to provide a comprehensive look at the phenomenon of online harassment and to advance the field toward effective human-oriented solutions.