Improved accountability is often called for as an element in improving health system performance. At first glance, the notion of better accountability seems straightforward, but it contains a high degree of complexity. If accountability is to be more than an empty buzzword, conceptual and analytical clarity is required. This article elaborates a definition of accountability in terms of answerability and sanctions, and distinguishes three types of accountability: financial, performance and political/democratic. An analytic framework for mapping accountability is proposed that identifies linkages among health sector actors and assesses capacity to demand and supply information and exercise oversight and sanctions. The article describes three accountability purposes: reducing abuse, assuring compliance with procedures and standards, and improving performance/learning. Using an accountability lens can: (1) help to generate a system-wide perspective on health sector reform, (2) identify connections among individual improvement interventions, and (3) reveal gaps requiring policy attention. These results can enhance system performance, improve service delivery and contribute to sound policymaking.