These techniques yield valuable insights into complex natural organic matter. Three research approaches to characterize natural organic matter (NOM) are available: (1) a nonperturbing approach, (2) an approach based on isolation–fractionation, and (3) an approach based on calculations. A nonperturbing approach has been used to assess the aromaticity and morphology of NOM. Characterization of isolated fractions has shown that the two main fractions—fulvic and hydrophilic acids—are in some points structurally different. Furthermore, reasonable estimates of several structural features of NOM have been obtained by analytical-constraints calculations. Each approach yields valuable insights into NOM—its source and nature in watersheds, how seasonal variations affect it, and its potential to form disinfection by-products. Each approach also yields insight into the relationship between NOM's structural and compositional properties.