Health systems worldwide are under pressure to deliver better care for more people from fewer resources. The global economic crisis has shrunk the resources available for healthcare but the growth in demand for care services continues unabated. "Learning Health Systems" is a novel health informatics paradigm that blends quality improvement methods with data science. The goal is to create an integrated health system which harnesses routinely-collected health data to learn from every patient, and feed the knowledge of “what works best” back to clinicians, public health professionals, patients, and other stakeholders to create cycles of continuous improvement. In this talk we dissect the new paradigm and explore its opportunities and challenges for data scientists.