Conservation and improvement of the environment all over the world is one of the main ecological problems facing mankind today. For many densely populated countries with a highly developed industry and intensive agriculture, one vital problem is to maintain a water supply of sufficient purity. Water management is a fairly complex, unremitting task requiring the cooperation of the many different bodies concerned — the water management authorities at all levels and the relevant research institutes. The growing quantity of organic matter in streams and reservoirs due to the spreading use of inorganic fertilizers has become the most urgent problem in connection with the quality of water. This is a worldwide problem, since growing demand for food can be satisfied only by more intensive agriculture using high quantities of fertilizer. These fertilizers are washed out of the soil into streams and reservoirs and seep into the ground water. It is essential to reconcile the mutually antagonistic demands of modern agriculture and water management. To do so, figures must be available on the amount of organic matter to be expected in the polluted water. A simple method is needed to elucidate the complex of factors which influence the growth and development of primary producer populations, and thus permit the assessment of their production potential. Of the available methods, the control of the trophic level of natural and polluted waters by means of algal assays appears to be a most promising approach. A thorough elaboration of this method and its general application in water quality control require an adequate understanding of the biological and physiological features of algal populations and communities. The selection of suitable species and the application of appropriate cultivation techniques are also essential for the successful utilization of these bio-assays. The present volume provides a summary of current knowledge, as presented at the colloquium on algal assays in Trebon in 1976.