A Web service that issues security tokens. A security token service makes assertions based on evidence that it trusts to whoever trusts it. To communicate trust, this service requires proof, such as a security token or set of security tokens, and it issues a security token with its own trust statement. (Note that for some security token formats, this can simply be a reissuance or cosignature.) In Active Directory Federation Services (ADFS), the Federation Service is a security token service.