The Natural Step framework for sustainability is 15 years old this year. The framework and the organisation, The Natural Step International, have been through many changes during that time, yet the fundamentals of this unique, science-based approach remain unchanged. TNS is now well-known around the world: there are teams established in ten countries, and growing activity in several others. In this Schumacher Briefing David Cook tells the story of the evolution of the organisation from its start in Sweden to the present day. Whilst the fundamentals of TNS remain the same, it has responded to the ever-evolving sustainability debate. Based upon science and systems thinking, the basic components of the TNS framework are laid out in the text. The track record of TNS has developed from its scientific approach to complex issues. It has also built its reputation from groundbreaking work done with several large corporations in Europe and America. In the Briefing, many successes are brought together for the first time - from work with small and medium sized enterprises, large companies, communities and the public sector. In each of the cases described, the Briefing highlights key learning processes and outcomes. The text contains answers to many questions commonly asked about The Natural Step. A major component of the Briefing is that it gives a deeper exposition of the power of systems thinking and its crucial relevance to leadership in the world today.