Electronic data sharing is a critical, but underappreciated, requirement for modern medical informatics systems. This capability is facilitated by acquisition of structured clinical data, but optimized only if this data is stored and transmitted using standardized representations. Most electronic medical record and clinical trials management systems are poorly suited for data sharing. In the near future, sharing clinical outcome data is likely to become very important, so these capabilities must be improved. In this article, basic concepts of electronic data sharing are reviewed and their use illustrated in a data-sharing project developed to support Hematopoietic Cell Transplantation reporting to a national database.