Preserving the meaning of information when exchanging electronic health record data (i.e., semantic interoperability) is critical for delivering safe patient care and leveraging standards-based clinical decision support.  Given that individuals often receive health care from more than one health system, integration of data from multiple sources is needed to ‘view’ a patient’s complete health record and avoid erroneous clinical decisions based on incomplete or inaccurate information, such as decisions that lead to performing unnecessary tests or giving a patient a drug to which they are known to be allergic. To date, the strategy for achieving semantic interoperability between the clinical systems of the Department of Defense (DoD) and the Veteran’s Administration (VA) has been to ‘map’ millions of data elements used in the respective EHRs to standard terminologies (e.g., SNOMED CT, LOINC, and RxNorm).  ‘Round trip testing’ of the mapped concepts has identified problems with the quality of the mappings for bidirectional use.  New strategies are required to achieve semantic interoperability to support safe patient care, both before and after the two organizations start using of a single vendor for their electronic health record systems. The use of logical definitions and terminology system extensions to manage concepts used in the delivery of care can overcome key challenges with the mapping strategy. (Achieving Semantic Data Interoperability)