Within the environmental sciences, screening broadly refers to a set of analytical techniques used to monitor levels of potentially hazardous organic compounds in the environment, particularly in tandem with mass spectrometry techniques. Such screening techniques are typically classified as either targeted, where compounds of interest are chosen before the analysis begins, or non-targeted, where compounds of interest are chosen at a later stage of the analysis. These two techniques can be organized into at least three approaches: target screening, using reference standards that are analogous to the target compound; suspect screening, which uses a library of cataloged data such as exact mass, isotope patterns, and chromatographic retention times in lieu of reference standards; and non-target screening, using no pre-existing knowledge for comparison before analysis. As such, target screening is most useful when monitoring the presence of specific organic compounds—particularly for regulatory purposes—which requires higher selectivity and sensitivity. When the number of detected compounds and associated metabolites needs to be maximized for discovering new or emerging environmental trends or biomarkers for disease, a more non-targeted approach has traditionally been used. However, the rapid improvement of mass spectrometers into more high-resolution forms, with increased sensitivity, has made suspect and non-target screening more attractive, either as stand-alone approaches or in conjunction with more targeted methods.