In recent years, the importance of Environmental, Social, and Governance (ESG) factors in shaping business sustainability and performance has gained significant attention. ESG is not only a measure of a company's social responsibility but also a crucial determinant of its long-term success. Despite the widespread recognition of ESG's importance, its impact on firm performance remains a subject of debate. In this paper, we systematically review and analyze the complex relationship between ESG and firm performance. We first introduce major ESG rating methods and agencies, followed by a synthesis of the empirical evidence on the relationship between ESG and corporate financial performance, with particular emphasis on the conflicting and inconclusive findings in the literature. Subsequently, we examine the mechanisms through which ESG practices influence firm performance, focusing on legitimacy, reputation, and corporate management as key pathways. Through this analysis, we aim to provide insights into how ESG can drive sustainable development and influence firm performance in the long run. Our study contributes to the ongoing academic discussions and practical advancements in the field of ESG by providing a structured approach to understanding the complex and multidimensional relationship between ESG and corporate performance and identifying future research directions.