Dissolved organic matter (DOM) plays a crucial role in freshwater health, influencing chemical, physical, and biological processes. DOM consists of thousands of distinct organic compounds from various sources, and its diversity, termed chemodiversity, is vital for ecosystem function. Four main lines of evidence show that different types of DOM affect ecosystem functioning. DOM from land can supply nutrients, influence algal blooms, affect contaminant toxicity, and alter microbial metabolism. DOM with more biolabile compounds can be quickly transferred to microbial biomass, affecting water clarity and carbon fluxes. Conversely, DOM with more biorefractory compounds may lead to higher carbon accumulation. High-resolution mass spectrometry is the state-of-the-art method for measuring chemodiversity, providing insights into DOM composition and its ecological roles. This approach can identify sources of contaminants and link them to specific pollution sources, such as agricultural runoff or wastewater. Measuring chemodiversity also helps determine the availability of trace metals like iron for biological uptake. Molecular-scale resolution of DOM can reveal harmful by-products formed during water treatment. Monitoring chemodiversity can adapt to different aspects of freshwater health, from drinking water quality to food web assessments. Climate change may alter DOM composition, affecting ecosystem health. Anthropogenic changes, such as land use and pollution, also modify DOM. Understanding chemodiversity is essential for protecting freshwater ecosystems. By characterizing DOM sources and predicting their mixing, chemodiversity can be mapped across large spatial and temporal scales. Manipulating DOM through vegetation management can influence downstream chemodiversity. Ultimately, understanding chemodiversity is key to protecting freshwater health and its surrounding catchments.Dissolved organic matter (DOM) plays a crucial role in freshwater health, influencing chemical, physical, and biological processes. DOM consists of thousands of distinct organic compounds from various sources, and its diversity, termed chemodiversity, is vital for ecosystem function. Four main lines of evidence show that different types of DOM affect ecosystem functioning. DOM from land can supply nutrients, influence algal blooms, affect contaminant toxicity, and alter microbial metabolism. DOM with more biolabile compounds can be quickly transferred to microbial biomass, affecting water clarity and carbon fluxes. Conversely, DOM with more biorefractory compounds may lead to higher carbon accumulation. High-resolution mass spectrometry is the state-of-the-art method for measuring chemodiversity, providing insights into DOM composition and its ecological roles. This approach can identify sources of contaminants and link them to specific pollution sources, such as agricultural runoff or wastewater. Measuring chemodiversity also helps determine the availability of trace metals like iron for biological uptake. Molecular-scale resolution of DOM can reveal harmful by-products formed during water treatment. Monitoring chemodiversity can adapt to different aspects of freshwater health, from drinking water quality to food web assessments. Climate change may alter DOM composition, affecting ecosystem health. Anthropogenic changes, such as land use and pollution, also modify DOM. Understanding chemodiversity is essential for protecting freshwater ecosystems. By characterizing DOM sources and predicting their mixing, chemodiversity can be mapped across large spatial and temporal scales. Manipulating DOM through vegetation management can influence downstream chemodiversity. Ultimately, understanding chemodiversity is key to protecting freshwater health and its surrounding catchments.