This paper aims to give the reader a comprehensive overview of what has been the main evolution in the transition from Socially Responsible Investments to the ESG approach. This first step provides the basis for contextualising the current rating system of the major players in the global market. RepRisk and Sustainalytics are the most specialised, MSCI the leading player. In fact, as a best practitioner, an analysis of the characteristics of each of the main types of indices is given, as well as their dynamic composition over time, with company examples useful as case studies.