Humic substances are widely distributed in the environment, occurring in soils and waters. They are the products of microbial degradation of dead vegetable and animal matter. They account for the major portion of dissolved organic matter. They are water soluble and have the ability to fix persistent hydrophobic pollutants such as PCB's, halogenated aliphatics and aromatics and heavy metal ions, rendering them water soluble. Humic substances play a very important role in the mobilization and transport of persistent pollutants. The chemical nature of humic and fulvic acids is discussed fully. The nature of humic-contaminant interaction is dealt with according to current knowledge. The implications of an understanding of these interactions to environmental problems are discussed. Profitable lines of future enquiry are delineated. A full bibliography is provided.