In 2007, the Department of Health and Human Services commissioned the Nationwide Health Information Network (NHIN) Trial Implementations project to demonstrate the secure exchange of data among health information exchange organizations around the country. The project's Core Services Content Work Group (CSCWG) developed the content specifications for the project. The CSCWG developed content specifications for a summary patient record and for eight use cases that were implemented in demonstration events in 2008. The CSCWG developed tools to represent the specifications and facilitate implementation. The experience revealed that, in general, the Health Information Technology Standards Panel (HITSP) constructs served as a suitable starting point for the development of content specifications for interoperability. The ability to adhere to specified terminologies still presents significant challenges. This paper describes the process of developing the content specifications and lessons learned.