In order to support semantic interoperability in eHealth systems, domain terminologies need to be carefully designed. SNOMED CT and the upcoming ICD-11 represent a new generation of ontology-based terminologies and classifications. The proposed alignment of these two systems and, in consequence, the validity of their cross-utilisation requires a thorough analysis of the intended meaning of their representational units. We present the ICD11 SNOMED CT harmonization process including: a) the clarification of the interpretation of codes in both systems as representing situations rather than conditions, b) the principles proposed for aligning the two systems with the help of a common ontology, c) the high level design of this common ontology, and d) further ontology-driven issues that have arisen in the course of this work.