Classified information is confidential material that a government, corporation, or non-governmental organisation deems to be sensitive information, which must be protected from unauthorized disclosure and that requires special handling and dissemination controls. Access is restricted by law, regulation, or corporate policies to particular groups of individuals with both the necessary security clearance and a need to know.

Classified information within an organisation is typically arranged into several hierarchical levels of sensitivity—e.g. Confidential (C), Secret (S), and Top Secret (TS). The choice of which level to assign a file is based on threat modelling, with different organisations have varying classification systems, asset management rules, and assessment frameworks. Classified information generally becomes less sensitive with the passage of time, and may eventually be reclassified or declassified and made public.

Governments often require a formal security clearance and corresponding background check to view or handle classified material. Mishandling or unlawful disclosure of confidential material can incur criminal penalties, depending on the nature of the information and the laws of a jurisdiction. Since the late twentieth century, there has been freedom of information legislation in some countries, where the public is deemed to have the right to all information that is not considered to be damaging if released. Sometimes documents are released with information still considered confidential redacted. Classified information is sometimes also intentionally leaked to the media to influence public opinion.

In many jurisdictions, for example, the United States and United Kingdom, Top Secret (TS) is the highest level of classified information. Prior to 1942, the United Kingdom and other members of the British Empire used Most Secret, but this was later changed to match the United States' category name of Top Secret in order to simplify Allied interoperability. The unauthorized disclosure of Top Secret information is expected to cause harm and be of grave threat to national security.

It is desired that no document be released which refers to experiments with humans and might have adverse effect on public opinion or result in legal suits. Documents covering such work field should be classified "secret".

Secret material is often regarded as causative of "serious damage" to national security if it were publicly available, although not as serious harm as in the case of Top Secret classification.

Confidential material is material that would cause "damage" or be prejudicial to national security if publicly available. It is used in the US since as early as 1936. A relatively recent revision of its definition is in Executive Order 13526.

Restricted material would cause "undesirable effects" if publicly available. Some countries do not have such a classification in public sectors, such as commercial industries. Such a level is also known as "Private Information". Such a level existed within the US Government during World War II but is no longer used.