A discussion of the cycles of nutrient elements will begin with phosphorus (P), because it is the most limiting nutrient, or key nutrient, in fresh water. The annual productivity of freshwater lakes is controlled more often by P than by any other nutrient or environmental factor. The forms of P and a general cycle will be described first and the more important processes will be indicated. Because of the importance of the P store in the sediments as a long-term source to the overlying water in lakes, even after external inputs have been curtailed, the reactions at the sediment-water interface will be discussed in detail. The principal processes involved in P cycling will be applied to laboratory and open-lake conditions by reviewing pertinent experimental results.