Countries may filter sensitive content on an ongoing basis and/or introduce temporary filtering during key time periods such as elections.In some cases the censoring authorities may surreptitiously block content to mislead the public into believing that censorship has not been applied.

Among the most popular filtering software programs is Smart Filter by Secure Computing in California, which was bought by Mc Afee in 2008.

Smart Filter has been used by Tunisia, Saudi Arabia, Sudan, the UAE, Kuwait, Bahrain, Iran, and Oman, as well as the United States and the UK.

In a 2012 Internet Society survey 71% of respondents agreed that "censorship should exist in some form on the Internet".

In the same survey 83% agreed that "access to the Internet should be considered a basic human right" and 86% agreed that "freedom of expression should be guaranteed on the Internet".

Blacklists may be produced manually or automatically and are often not available to non-customers of the blocking software.

Blocking or filtering can be done at a centralized national level, at a decentralized sub-national level, or at an institutional level, for example in libraries, universities or Internet cafes.

Pseudonymity and data havens (such as Freenet) protect free speech using technologies that guarantee material cannot be removed and prevents the identification of authors.

Technologically savvy users can often find ways to access blocked content.

Nevertheless, blocking remains an effective means of limiting access to sensitive information for most users when censors, such as those in China, are able to devote significant resources to building and maintaining a comprehensive censorship system.

Technical censorship techniques are subject to both over- and under-blocking since it is often impossible to always block exactly the targeted content without blocking other permissible material or allowing some access to targeted material and so providing more or less protection than desired.

The companies sell products that are liable to be used by governments to violate human rights and freedom of information. lawsuit filed in May 2011, Cisco Systems is accused of helping the Chinese Government build a firewall, known widely as the Golden Shield, to censor the Internet and keep tabs on dissidents. Cisco is also accused of aiding the Chinese government in monitoring and apprehending members of the banned Falun Gong group.