In this article, principles of phosphorus retention and phosphorus release at the sediment-water interface in lakes are reviewed. New results and hypotheses are discussed in relation to older models of phosphorus exchange between sediments and water. The fractional composition of sedimentary phosphorus is discussed as a tool for interpretation of different retention mechanisms. Special emphasis is given to the impact ofbiological, particularly microbial, processes on phosphorus exchange across the sediment-water interface and to the significance of biologically induced CaCO, precipitation to phosphorus retention in calcareous lakes,