Although diatom analysis has been used in archaeological science for over 40 years, it is still an under-used technique. In this paper, the basis of the technique is explained and the literature on diatoms in archaeology is reviewed. Diatoms are good indicators of a range of water quality variables, but emphasis here, reflecting their dominant use so far in archaeology, is placed on their value as indicators of water salinity. A number of case studies are described from different parts of Europe, demonstrating the use of the technique in the location of prehistoric settlement sites along former shorelines, in reconstructing salinity and tidal regimes in estuaries, and in the location of clay sources for pot manufacture. The potential of diatom analysis in contributing to questions of pottery typology is also indicated. Although diatom taxonomy remains very difficult and specialist analysis will always be required, when there is an archaeological need to ask questions relating to the presence, absence, level, pollution, productivity, acidity or salinity of a waterbody on or near to an excavation, diatom analysis should be considered. -- AATA