The authors are conducting a project to evaluate how wastewater-derived disinfection byproducts (DBPs) and their precursors contribute to DBPs in drinking water. Here, a field survey of many different types of wastewater treatment plants documents that they are sources of halogenated DBPs, if chlorine disinfection is practiced, and DBP precursors in all cases. Because the level of ammonia in treated wastewater usually is high, the addition of chlorine typically forms combined chlorine (chloramines), which minimizes the instantaneous formation of most halogenated DBPs. However, nitrosamines (e.g, N-nitrosodimethylamine [NDMA]) often are formed. Treated wastewater contains residual drinking water natural organic matter (NOM). Although the dissolved organic carbon (DOC) concentration of the effluent organic matter (EfOM) is relatively high compared to drinking water, the humic content of the EfOM - based on specific ultraviolet absorbance (SUVA) - is relatively low. Waters low in SUVA tend to be less reactive with chlorine and typically form less trihalomethanes (THMs) and other DBPs per unit DOC. The presence of THM precursors and, in some cases, haloacetic acid precursors correlates best with SUVA. Preliminary data suggest that the formation of certain nitrogenous DBPs (e.g., haloacetonitriles and nitrosamines) or the presence of their precursors in treated wastewater is significant. The presence of precursors for some nitrogenous DBPs correlates with the level of dissolved organic nitrogen.