Selenium is a constituent of petroleum, present at 0.03 – 1.4 parts per million. Crude oils from the San Joaquin Valley of California have among the highest selenium concentrations. Because of its chemical similarity to sulfur, selenium follows sulfur through the petroleum refining process to the sour water strippers, where sulfur is stripped overhead while selenium remains in the stripper bottoms. Selenium in sour water stripper bottoms appears to be in reduced forms such as selenide (Se2-). The bottoms are discharged to the refinery wastewater treatment system, where some selenium is removed by biological treatment. The remaining selenium gets oxidized to selenite (SeO32-) and selenate (SeO42-). Selenium discharged to receiving waters is an environmental concern because of bioaccumulation by algae and subsequent concentration by other organisms higher in the food chain. Removal of selenium is dependent on its speciation and on concentrations of other contaminants in the wastewater. Iron coprecipitation, in which iron salts are added to the wastewater and selenium is removed on the surface of precipitated iron hydroxide, is most effective at removing selenite. Selenate is more loosely bound to iron hydroxide than selenite, and sulfate competes with selenate for adsorption sites, so selenate removal by iron coprecipitation is poor. Iron coprecipitation produces large quantities of sludge, which can be a hazardous waste. Iron reduction processes appear to be effective at removing various species of selenium. Other removal technologies such as ion exchange and anaerobic biological reduction have been tested. Capital costs for removing selenium can often be reduced by treating concentrated streams within the refinery rather than the final refinery effluent.