The EMI project is responding to the challenges in the Grid credential management with a new general purpose service, the Security Token Service (STS). STS is a service that can be used for transforming an existing security token into another security token format. As the supported security token formats include username/password, SAML assertion, X.509 certificate and VOMS proxy certificate, a user holding any type of such credentials is able to convert them into another supported type, for instance in order to access Grid services. By enabling the token transformation, STS can establish a trust relationship between different security and application domains. The presentation contains a short introduction to the technology, description of the most important use cases and the current status of the implementation.