The Dead Sea is a hypersaline terminal lake with its surface water at -417 m below mean sea level. The Dead Sea water is dense, with 30% dissolved solids - one of the highest natural average salinities on Earth (276 g/kg). It has a unique composition of minerals, with KCl being the major component in comparison to NaCl in the oceans. Being such a highly saline terminal lake, the Dead Sea is characterized by extreme conditions including low pH values, restricted biological excise, and thus represents a unique "environmental laboratory" for the aquatic optical studies. Only the unicellular flagellate green alga (Dunaliella parva), several types of the halophilic Archaea (archaeobacteria) and eubacteria (bacteria) exist, and usually in very low concentration. Only when the upper water layers of the lake become diluted with fresh water after massive rain floods, do biological component show significant concentrations. In the rest of the time, the lake waters are rich with various sediments and organic matter, monitoring of which can essentially improve our understanding of ecological state of the lake. Modern satellite investigations can improve our understanding of the global arge-scale optical variations in the lake, but they should be supported by reliable bio-optical field observations in the Dead Sea. The first regular optical measurements (of a spectrally-averaged transmittance) have been carried out during the period from the 1960s to 1980s by Dr. David Anati. However, these measurements have not been analyzed in details. We present results of processing of these historical data in the form of a vertical gradient of downwelling attenuation coefficient, and show that there is an essential relation of this optical property with the Dunaliella parva content in the lake. Similarly, we show that the spectral reflection function is defined primarily by the suspended matter concentration, and, consequently, it can be used for in situ or remote-sensing estimation of the latter. A simple atmospheric transmittance model above the Dead Sea has been developed from the historical data of the incoming solar irradiance. Using the reciprocity principle for transmittance make potentially possible a development of atmospheric correction algorithm, which will take into account a very dense atmosphere. Such an algorithm along with the in situ relationships derived can be used for monitoring the lake and ecological forecasting.Sokoletsky, L., H. Gildor, and E. Boss, 2006. Bio-Optical Properties of the Dead Sea. 2006 Spring Ocean Sciences Meeting.