Since 2016, national governments around the world have worked towards aligning their national development plans to the 17 Sustainable Development Goals (SDGs) [1]. Meeting the SDGs requires different strategies in different countries, and policy makers have to address, among others, questions such as: How many resources are needed to achieve the SDGs? How to best distribute investment across different areas? Where to invest first? How to finance such investment? The iSDG model has been designed and developed to support decision-makers addressing such questions.

The iSDG model enables policy makers and planning officials at all levels of governance to understand the interconnectedness of policies designed to achieve the SDGs and test their likely impacts before adopting them. The iSDG model is constructed starting from the well-vetted, time-tested and validated Threshold21 (T21) model, covers all the SDGs, and also supports a better understanding of the interconnections of the goals and targets in order to develop synergetic strategies to achieve them. The iSDG model simulates the fundamental trends for SDGs until 2030 under a business-as-usual scenario, and supports the analysis of relevant alternative scenarios. The model also traces the trends beyond the SDGs’ time span all the way up to 2050.

The iSDG model is a System Dynamics based tool that has been designed to support national development planning. iSDG is structured to analyze medium- and long-term development issues at a national level. The model integrates into a single framework the economic, social, and environmental aspects of development planning. The level of aggregation used makes it ideally suited to assess resource allocation issues across different ministries. iSDG is conceived to complement budgetary models, sectoral models, and other short- to medium-term planning tools by providing a comprehensive and long-term perspective on development.

The iSDG is useful at four levels for SDG planning. First, it allows analyzing how – under business as usual conditions – the country would progress towards each of the 17 SDGs. Such analysis provides an initial overview of the areas that require more attention from policy makers. Second, the high level of interconnectedness among goals in the model allows for building a shared understanding among stakeholders of how development in each area affects (and might be necessary for) developments in other areas. Such an understanding provides important insights on the fundamental leverage points in the system – i.e. points of intervention that can lead to rapid and positive change. Third, the model supports the simulation of a variety of policies addressing each of the 17 Goals, in isolation and in combination with others, to understand their relevance and possible synergies. Finally, based on such analysis, a coherent SDG strategy can be developed, and the financial needs for its implementation can be assessed.

The T21 model on which iSDG is based has evolved over two decades as applications to individual countries were developed, tested, modified, and refined. Clients have included Ministries of Finance, Planning and other line ministries; UN agencies, the World Bank, and other international institutions; private sector and academic institutions. To date, T21 has been customized for over forty countries around the world, and the model’s basic structure - with customization - was found to represent countries as diverse as Malawi, Bangladesh, China, Italy, and the United States.

As applications of the model grow, restructuration of certain sectors such as health and nutrition, education, energy consumption, generation and supply, water withdrawal and supply, etc. Based on such experience, the T21, and now the iSDG has been continuously improved and all the most valuable extensions retained in the ever-evolving core model. This document represents the official documentation of the T21-iSDG model, and is meant for the general public. This documentation does not include highly technical modeling aspects, nor all the references to the excellent research work on which the iSDG model has been built. For simplicity and readability, we indicate only the most relevant sources of information.