Wikipedia's blackout protest against SOPA explained
18 January 2012 Last updated at 12:17 GMT
Wikipedia has taken its English-language site offline as part of protests against proposed anti-piracy laws in the US.
The protest began at midnight on Tuesday, Eastern Time in the United States.
The user-generated news site Reddit and the blog Boing Boing are also taking part in the "blackout".
Wikipedia, which attracts millions of hits every day, is opposed to the US Stop Online Piracy Act (Sopa) and Protect Intellectual Property Act (Pipa) being debated by Congress.
The BBC's Technology Correspondent Rory Cellan-Jones discussed the reasons behind the protest.