Health information systems represent a complex series of efforts to collect, analyze, and report information systematically and sustainably and to derive knowledge and evidence serving various purposes with the ultimate aim of improving health. The terminology used in relation to health information lists notions such as health data, health information or health statistics, and knowledge or evidence. The history of health statistics offers a picture of how the diversity of data has increased over time and how better health information has allowed scientists and health practitioners to reach better health outcomes. Providing an evidence base for overall policy and resource allocation, ensuring access (especially for vulnerable groups), monitoring the quality of care provided by the health system, understanding overall public health and the factors affecting it in different settings and ensuring transparency and accountability are the main purposes of health information systems. Examining the interactions between data used in national and international contexts is important in view of gaining better efficiency of health interventions. Considering the social determinants of health and their intersectoral nature prompts the need to gather data generated in other areas and sectors other than health. Registration systems such as civil registration and vital statistics are an important source of health information. Depending on the context, resources such as registries, notification systems, health surveys, nonhealth sector sources are also employed to obtain health statistics. Data integration is instrumental in monitoring health policy, monitoring and understanding of diseases, and better tracking individual health and well-being. However, ethical considerations and policy precautions are vital in data collection, storage, use and destruction, security and consent.