This technical guide provides an in-depth overview of National Nutrition Information Systems (NNIS), developed by the WHO-UNICEF Technical Expert Advisory Group on Nutrition Monitoring. It outlines the fundamental principles, practices, and processes essential for collecting, storing, organizing, analyzing, and disseminating nutrition-related data across various sectors such as health, agriculture, and education. The guide emphasizes the strategic value of robust nutrition information systems in improving health and wellbeing by supporting informed decision-making and enhancing nutrition programs. It discusses key components like human resources, data, processes, and technology required to build and maintain an effective NNIS. The publication aims to assist policymakers, nutrition program managers, and data analysts in strengthening their country's nutrition information infrastructure.