Some users may misbehave under the cover of anonymity by, e.g., defacing webpages on Wikipedia or posting vulgar comments on YouTube. To prevent such abuse, we have explored various anonymous credential schemes to revoke access for misbehaving users while maintaining their anonymity. Our latest scheme, PERM, supports millions of user sessions and makes ‘reputation-based blacklisting’ practical for large-scale deployments. Our paper on PERM was presented at the 19th ACM Conference on Computer and Communications Security. For more details see the accountable anonymity project page.