The chapter analyses ESG indicators, focusing on how they are constructed, detected and used in corporate valuations and investment processes. After an overview of the main data providers and metrics used to measure environmental, social and governance performance, the text explores the growing diffusion of equity and bond ESG indices, highlighting their function in financial markets. The role of ESG indicators in the definition of green bonds, sustainability-linked bonds and other sustainable finance instruments is explored in depth. Particular attention is paid to the problems of methodological heterogeneity among ESG rating providers, which hinder comparability and the benchmarking function. The chapter highlights the implications of such divergences for institutional investors and the overall credibility of ESG instruments, also highlighting the risks of greenwashing. Finally, the recent regulatory developments at the European level are discussed, which aim to strengthen the transparency and reliability of ESG indicators, promoting the adoption of more standardised criteria consistent with the objectives of sustainable finance. The chapter thus offers a critical and updated overview of the strategic role of ESG indicators in the evaluation, disclosure and capital allocation processes.