In September 1978 the Engineering and Technological Aspects (ETA) Committee expressed its concerns to the Great Lakes Science Advisory Board that efforts to control inputs of pollutants to the Great Lakes, such as heavy metals and, in particular, phosphorus, were not directed at biologically available forms, but rather at the total inputs of the pollutants. The Committee requested and received concurrence of theBoard to investigate the concept of controlling biologically available forms of phosphorus. Two major activities took place in carrying out this task. ETA Committee members participated in a review of phosphorus availability presented at the IJC/Cornell University Conference on "Phosphorus Management Strategies for the Great Lakes". The major paper on phosphorus availability presented at the conference and published in the proceedings was prepared by Lee et al. (1). Secondly, a state-of-the-art report on "Biologically Available Phosphorus" (2) was developed by a subcommittee of the ETA Committee. This report was based primarily on information received at a meeting of experts convened by the Committee on December 8, 1978 in Chicago. Those who attended this meeting are listed on page 29 of this report. Based on the above reports, and other relevant documents, the ETA Committee has prepared this summary report to answer the following questions with respect tophosphorus availability: 1) What forms of phosphorus are biologically available? 2) What analytical techniques are available to measure different forms of phosphorus and how do they compare with respect to their estimation of available phosphorus? 3) What are the differences in the proportion of biologically available phosphorus in total phosphorus inputs from various sources? and 4) What transformations in chemicalforms of phosphorus take place in lakes and what effect do they have onconsiderations of phosphorus availability? The Committee reached several conclusions based on its review of the question of phosphorus availability and in this summary report, discusses the implications for developing further strategies to control phosphorus inputs to the Great Lakes.