Image-based sexual abuse—the non-consensual creation, taking, sharing, or threat to share nude or sexual images—is a growing global issue, with research demonstrating that this form of technology-facilitated sexual violence is common and increasing. Image-based sexual abuse continues to occur partly because individuals actively or passively support inequality, discriminatory attitudes, or violence; remain silent; and/or tolerate the abuse and its underlying causes. Intervention by people who witness image-based sexual abuse behaviours, referred to as bystander intervention, has the potential to reduce the extent and impact of image-based sexual abuse, problematic attitudes, and harms. In this regard, bystanders are an untapped resource and have a potentially greater role to play in the prevention of image-based sexual abuse. In this chapter, we summarise the key definitions and literature relevant to image-based sexual abuse. We also conceptualise bystander intervention and provide an overview of the effectiveness of bystander intervention programs in the context of sexual violence. This chapter concludes with an outline of the book.