Abstract: :This review starts with the description of the quantitative significance of dissolved organic material in general and dissolved humic substances (HS)in particular in various ecosystems.Despite their high quantities, the knowledge about the role of HS is still very low and full of old, but still recycled paradigms.HS are thought to be inert or at least refractory and too large to be taken up by aquatic organisms.Instead, I present evidence that dissolved HS that mainly derives from the terrestrial environment, are taken up and directly and/or indirectly interfere with freshwater organisms and, thus, structure biocenoses. Relatively well known is in the meantime the fuelling function of allochthonous HS, which, upon irradiation, release fatty acids, which serve as substrates for microbial growth.This is an indirect effect of HS.Microbes, in turn, are food for mixotrophic algae and (heterotrophic)zooplankton.Thus, non-eutrophicated freshwaters are net-heterotrophic, meaning that respiration exceeds primary production.Furthermore, model calculations exemplify that only a very small portion of the terrestrial production is sufficient to cause net-heterotrophy in these freshwater bodies.But, recent papers show also that due to different stoichiometries the maximal plankton biomass production with algae or mixotrophs is higher than with bacteria. Very recently, several direct effects of HS have been elucidated.Among them are :induction of chaperons (stress shock proteins), induction and modulation of biotransformation enzymes, modulation (mainly inhibition)of photosynthetic oxygen release of aquatic plants, production of an internal oxidative stress, modulation of the offspring numbers in the nematode Caenorhabditis elegans, feminization of fish and amphibs, interference within the thyroid system, and action as chemical attractant to C.elegans. We are still in the phase of identifying the various physiological, biochemical, and molecular-biological effects.Hence, the ecological and ecophysiological significance of these HS-mediated effects still remain somewhat obscure.Nevertheless, HS appear generally to have an impact on the individual as well as on the community and even ecosystem level comparable to that of, for instance, nutrients.