The growing prominence of environmental, social, and governance (ESG) factors in global capital markets has raised fundamental questions about how sustainability can be systematically integrated into investment valuation. While academic literature provides extensive evidence on the financial relevance of ESG, challenges persist due to fragmented disclosure frameworks, divergent rating methodologies, and the absence of standardized integration practices. This thesis addresses these challenges by exploring how ESG metrics can be embedded into traditional valuation models in a manner that is both replicable and strategically meaningful. The research adopts a dual-lens methodology. A qualitative synthesis reviews the main ESG frameworks (GRI, SASB/ISSB, TCFD, OECD, UN PRI…) and rating providers (MSCI, Sustainalytics, Refinitiv, CDP…), mapping their conceptual foundations and methodological divergences.