Formal modeling of clinical content that can be made available internationally is one of the most promising pathways to semantic interoperability of health information. Drawing on the extensive experience from openEHR archetype research and implementation work, we present the latest research and development in this area to improve semantic interoperability of Electronic Health Records (EHRs) using openEHR (ISO 13606) archetypes. Archetypes as the formal definition of clinical content need to be of high technical and clinical quality. We will start with a brief introduction of the openEHR architecture followed by presentations on specific topics related to the management of a wide range of clinical knowledge artefacts. We will describe a web-based review process for archetypes that enables international involvement and ensures that released archetypes are technically and clinically correct. Tools for validation of archetypes will be presented, along with templates and compliance templates. All this in combination enables the openEHR computing platform to be the foundation for safely sharing the information clinicians need, using this information within computerized clinical guidelines, for decision support as well as migrating legacy data.