Analysis of sediment diatom and chrysophyte assemblages is the best technique currently available for inferring past lake water pH trends. Use of the approach for assessing the ecological effects of acidic deposition is increasing rapidly. As of August 1989, sediment core inferred pH data existed for at least 150 lakes in North America and cores from about 100 more lakes are being analyzed. Equations for inferring past pH are based on at least 15-20 calibration data-sets involving about 700 lakes. Palaeolimnological studies indicate that recent acidification has been caused by acidic deposition in the Adirondack Mountains (New York), northern New England, Ontario, Quebec and the Canadian Atlantic provinces. With the exception of one lake, no acidification trends were observed in regions currently receiving low deposition of strong acids (e.g. Rocky Mountains and Sierra Nevada in the western United States).