A credential is a piece of any document that details a qualification, competence, or authority issued to an individual by a third party with a relevant or de facto authority or assumed competence to do so.

Examples of credentials include academic diplomas, academic degrees, certifications, security clearances, identification documents, badges, passwords, user names, keys, powers of attorney, and so on. Sometimes publications, such as scientific papers or books, may be viewed as similar to credentials by some people, especially if the publication was peer reviewed or made in a well-known journal or reputable publisher.

A person holding a credential is usually given documentation or secret knowledge (e.g., a password or key) as proof of the credential. Sometimes this proof (or a copy of it) is held by a third, trusted party. While in some cases a credential may be as simple as a paper membership card, in other cases, such as diplomas, it involves the presentation of letters directly from the issuer of the credential its faith in the person representing them in a negotiation or meeting.

Counterfeiting of credentials is a constant and serious problem, irrespective of the type of credential. A great deal of effort goes into finding methods to reduce or prevent counterfeiting.In general, the greater the perceived value of a credential, the more significant the problem of counterfeiting, and the greater the lengths to which the issuer must go to prevent fraud.

In diplomacy, credentials, also known as a letter of credence, are documents that ambassadors, diplomatic ministers, plenipotentiaries, and chargés d'affaires provide to the government to which they are accredited, for the purpose, chiefly, of communicating to the latter the envoy's diplomatic rank. It also contains a request that full credence be accorded to his official statements. Until his credentials have been presented and found in proper order, an envoy receives no official recognition.

The credentials of an ambassador or minister plenipotentiary are signed by the head of state, those of a chargé d'affaires by the foreign minister. Diplomatic credentials are granted and withdrawn at the pleasure of the issuing authority, based on widely varying criteria. A receiving government may reject a diplomat’s credentials by declining to receive them, but in practice this rarely happens.

In medicine, the process of credentialing is a detailed review of all permissions granted a medical doctor, physician assistant or nurse practitioner at every institution at which he or she has worked in the past, to determine a risk profile for them at a new institution. It vets the practitioner for both receiving practice insurance and the ability to bill to insurance for patient care. As well, it certifies legal and administrative body requirements, such as the Joint Commission.

Medical practitioners must also have credentials in the form of licenses issued by the government of the jurisdictions in which they practice, which they obtain after suitable education, training, and/or practical experience. Most medical credentials are granted for a practice specific group. They may also be withdrawn in the event of fraud or malpractice by their holders. Typically they require continuing education validation and renewal to continue practice.