The CheckUser tool is used by a small group of trusted Wikipedia users (called checkusers). The tool allows its users to determine from Wikipedia's servers the IP addresses used by a Wikipedia user account, as well as other technical data stored by the server about a user account or IP address.

Logo for the Wikipedia CheckUser tool and the checkuser team

Wikipedia's checkuser team uses the tool to establish whether two or more accounts are being operated by one individual or group of people, and then to protect Wikipedia against disruptive or abusive behaviour.

Checkusers are able to view a list of all IP addresses used by a user account to edit the English Wikipedia, a list of all edits made by an IP (which includes edits by accounts), or a list of all user accounts that have used an IP address. They may also view a log of CheckUser actions at Special:CheckUserLog.

CheckUser data is never accessed or released except in accordance with the Wikimedia Foundation's (WMF) privacy policy and the additional restrictions placed by this policy. After completing an investigation with the CheckUser tool, most checkusers release their findings in generalised form (such as with a statement that "account A is, is not, or may be the same as accounts B or C"); on the English Wikipedia, the release of actual private data (like "account D is connecting through IP address") is exceptionally rare. Conclusions derived from CheckUser data have limited usefulness, and a negative finding by a checkuser rarely precludes obvious sock-puppetry.

On the English Wikipedia, CheckUser access is entrusted to a restricted number of trusted users who can execute CheckUser inquiries at their own discretion. The permission is granted by Wikipedia's Arbitration Committee, after community consultation and vetting of the editor by the committee's members and the functionary team. While there is no formal requirement that checkusers also be administrators, the Arbitration Committee has traditionally restricted applications to users who are currently administrators. Checkusers must be 18 years of age or older and have signed the Wikimedia Foundation's confidentiality agreement for nonpublic information before being appointed. The use of the CheckUser tool on the English Wikipedia is monitored and controlled by the Arbitration Committee, and checkusers may have their permissions revoked by the Arbitration Committee for misuse or abuse of the tool.

This policy supplements the global checkuser policy and applies only to the English Wikipedia.