This textbook begins with an introduction to the US healthcare delivery system, its many systemic challenges and the prior efforts to develop and deploy informatics tools to help overcome those problems. It goes on to discuss health informatics from an historical perspective, its current state and its likely future state now that electronic health record systems are widely deployed, the HL7 Fast Healthcare Interoperability standard is being rapidly accepted as the means to access the data stored in those systems and analytics is increasing being used to gain new knowledge from that aggregated clinical data.  It then turns to some of the important and evolving areas of informatics including population and public health, mHealth and big data and analytics.  Use cases and case studies are used in all of these discussions to help readers connect the technologies to real world challenges. Effective use of informatics systems and tools by providers and their patients is key to improving the quality, safety and cost of healthcare.  With health records now digital, no effective means has existed for sharing them with patients, among the multiple providers who may care for them and for important secondary uses such as public/population health and research.  This problem is a topic of congressional discussion and is addressed by the 21st Century Cures Act of 2016 that mandates that electronic health record (EHR) systems offer a patient-facing API.  HL7’s Fast Healthcare Interoperability Resources (FHIR) is that API and this is the first comprehensive treatment of the technology and the many ways it is already being used.  FHIR is based on web technologies and is thus a far more facile, easy to implement approach that is rapidly gaining acceptance. It is also the basis for a ‘universal health app platform’ that literally has the potential to foster innovation around the data in patient records similarto the app ecosystems smartphones created around the data they store.  FHIR app stores have already been opened by Epic and Cerner, the two largest enterprise EHR vendors.  Provider facing apps are already being explored to improve EHR usability and support personalized medicine. Medicare and the Veteran’s Administration have announced FHIR app platforms for their patients.  Apple’s new IOS 11.3 features the ability for consumers to aggregate their health records on their iPhone using FHIR. Health insurance companies are exploring applications of FHIR to improve service and communication with their providers and patients. SureScripts, the national e-Prescribing network, is using FHIR to help doctors know if their patients are complying with prescriptions.  This textbook is for introductory health informatics courses for computer science and health sciences students (e.g. doctors, nurses, PhDs), the current health informatics community, IT professionals interested in learning about the field and practicing healthcare providers.  Though this textbook covers an important new technology, it is accessible to non-technical readers including healthcare providers, their patients or anyone interested in the use of healthcare data for improved care, public/population health or research.