This document provides an overview of health systems and their development and strengthening. It defines a health system and its key goals of good health outcomes, responsiveness, and fairness in financing. The six building blocks of a health system are described as service delivery, health workforce, information, medical products/vaccines/technologies, financing, and leadership/governance. Health system strengthening is defined as initiatives that improve one or more of these functions to enhance access, coverage, quality or efficiency. The document discusses challenges faced by health systems and some opportunities to address them.