The number of the more than two million organic chemicals that is detected in a sample of water is related to the sensitivity of the measurement technique: as the detection level decreases an order of magnitude, the number of compounds detected increases accordingly. We must determine which ones are present in concentrations significant enough to pose a hazard to human health. Fifty-five hundred observations of compounds actually observed in water revealed that many compounds must be considered in analyzing environmental samples; some compounds occur frequently, but many occur only sporadically. To determine which compounds are causing health problems (already observed or not yet observed) a survey of the entire nation is needed. Because so many compounds must be considered, a highly comprehensive analysis is obviously required. An approach using concentration by resin accumulators, solvent extraction, gas chromatography-mass spectrometry, and computerized data analysis gives hope for development of such a comprehensive analytical scheme, but some information gaps still exists.