The chapter discusses the role of dissolved organic matter (DOM) in maintaining the health of freshwater ecosystems, particularly lakes and rivers. DOM, composed of thousands of organic compounds from various sources, is crucial for ecosystem functioning. The authors propose that chemodiversity, the variety of chemical compounds in DOM, is a key factor in assessing and restoring freshwater health. Four main lines of evidence support this hypothesis: DOM from land can supply nutrients to food webs, influence the toxicity of contaminants, alter microbial metabolism, and modify the environment by affecting heat penetration in deep waters. High-resolution mass spectrometry is highlighted as the most advanced method for measuring chemodiversity, allowing for the identification of individual compound characteristics and their ecological roles. This approach provides insights into the determinants of freshwater health, such as the sources of contaminants and the impact of climate change on DOM composition. The authors also discuss the benefits of molecular-scale resolution in identifying harmful by-products of drinking water treatment and in tracing the sources of toxicants. The chapter concludes by emphasizing the need for monitoring and manipulating DOM at the catchment scale to protect freshwater ecosystems. It suggests that understanding the determinants of ecosystem health and the impacts of anthropogenic changes on chemodiversity is essential for effective conservation efforts.The chapter discusses the role of dissolved organic matter (DOM) in maintaining the health of freshwater ecosystems, particularly lakes and rivers. DOM, composed of thousands of organic compounds from various sources, is crucial for ecosystem functioning. The authors propose that chemodiversity, the variety of chemical compounds in DOM, is a key factor in assessing and restoring freshwater health. Four main lines of evidence support this hypothesis: DOM from land can supply nutrients to food webs, influence the toxicity of contaminants, alter microbial metabolism, and modify the environment by affecting heat penetration in deep waters. High-resolution mass spectrometry is highlighted as the most advanced method for measuring chemodiversity, allowing for the identification of individual compound characteristics and their ecological roles. This approach provides insights into the determinants of freshwater health, such as the sources of contaminants and the impact of climate change on DOM composition. The authors also discuss the benefits of molecular-scale resolution in identifying harmful by-products of drinking water treatment and in tracing the sources of toxicants. The chapter concludes by emphasizing the need for monitoring and manipulating DOM at the catchment scale to protect freshwater ecosystems. It suggests that understanding the determinants of ecosystem health and the impacts of anthropogenic changes on chemodiversity is essential for effective conservation efforts.