Laboratories have a strong, urgent need to assure the quality of their analytical results. Quality control (QC) is a key element in ensuring reliable results, because it requires both monitoring of the process and elimination of the causes of unsatisfactory performance. QC involves the use of blanks, certified reference materials (CRMs), duplicate samples, blind samples, and calibrating standards or spiked samples, among others. QC criteria are discussed for environmental monitoring of organic micro-contaminants in water. In addition, quality criteria in the identification and confirmation of target analytes are considered.