In this Perspective, the authors introduce a framework for internet users to categorize the virtual platforms they use and to understand the likelihood that they may encounter extreme content online. The authors first provide a landscape of the online extremist “ecosystem,” describing how the proliferation of messaging forums, social media networks, and other virtual community platforms has coincided with an increase in extremist online activity. Next, they present a framework to describe and categorize the platforms that host varying amounts of extreme content as mainstream, fringe, or niche. Mainstream platforms are those for which only a small portion of the content would be considered inappropriate or extreme speech. Fringe platforms are those that host a mix of mainstream and extreme content—and where a user might readily come across extreme content that is coded or obscured to disguise its violent or racist underpinning. Niche platforms are those that openly and purposefully cater to an extreme audience.