Blockchain is a powerful technology for enabling secure data sharing and access between multiple parties. This is a major challenge in digital health, where the privacy and security of medical data is paramount, but where improving the quality of care cannot happen without more coordination in management of patient data across the healthcare system and the ability to apply analytics to population level medical data. In short, blockchain can help digital health by making it easier to share data securely, with patient consent, across very fragmented healthcare systems.