The drainage of agricultural wastewater from the rich San Joaquin Valley — a problem that has vexed farmers, scientists, and politicians for many years — reached a climax early this year, when a halt was ordered in the delivery of federal irrigation water to 42,000 acres of land in the Westlands area of the Valley. Behind this action was the detection of high levels of selenium in Kesterson Reservoir, terminus for the 80-mile-long San Luis Drain, which carries saline wastewater from the Westlands Irrigation District to Kesterson. Built in 1971, the 12 shallow evaporation ponds at Kesteron supported a variety of fish and wildlife. Mortalities and deformities attributed to accumulated selenium attracted national attention. In this article, Dr. Richard Burau, Professor of Soil Chemistry in the Department of Land, Air, and Water at UC Davis, reviews what is known about selenium and how it enters the food chain.