In addition to water recycling and reclamation programs, indirect potable reuse of wastewater has occurred over the past few decades, which will likely increase in the future as upstream wastewater treatment plants (WWTPs) discharge water into rivers or lakes that serve as downstream drinking water supplies. Indirect potable reuse can be defined as any watershed for a drinking WTP (DWTP) that contains point source discharges of wastewater; septic tanks adjacent to rivers may also alter the quality of the surface water. Drought and competing instream demands may result in <10- to >50-percent contribution of treated wastewater towards the stream flow. Attention has focused on pharmaceuticals and endocrine disruptors, but WWTPs are also sources of disinfection byproducts (DBPs), if chlorine disinfection is practiced, and DBP precursors. Biological wastewater treatment takes one of two general forms: suspended growth (biofloc) systems (e.g., activated sludge), and attached growth (biofilm) systems (e.g., trickling filter). Depending on operational conditions, both can operate as partial or complete nitrifying processes. Increased levels of nitrification decrease the concentrations of ammonia and organic nitrogen (amino) compounds. Nitrification transforms ammonia and organic nitrogen to nitrate. Suspended growth systems under anoxic conditions can denitrify (convert nitrate primarily to nitrogen gas). Treated wastewater (effluent organic matter [EfOM]) has been shown to be a source of precursors for a wide range of DBPs (trihalomethanes [THMs], haloacetic acids [HAAs], haloacetonitriles [HANs], and nitrosamines). The objective of this study was to evaluate the contribution of treated wastewater to DBP formation in drinking water supplies. The authors conducted a full-scale survey of approximately 20 WWTPs in the U.S. (in the west, southwest, the mountain region, south central, midwest, and northeast). WWTPs were sampled that used a range of treatment processes (oxidation ditch, aerated lagoon, trickling filters, activated sludge, nitrification/denitrification, soil aquifer treatment [SAT], powdered and/or granular activated carbon [PAC, GAC], membrane bioreactor [MBR], reverse osmosis [RO], or various combinations). For most of the study sites, samples were collected at the WWTPs and downstream DWTPs, effluent-impacted rivers or monitoring wells. Some of the WWTPs in this study had sequential and/or parallel treatment processes for which separate samples were collected. For example, the secondary treatment process at one WWTP included trickling filters and solids contactors (no nitrification [NH3-N >10 mg/L]), which were followed by nitrifying trickling filters for ammonia removal. Another WWTP had two parallel treatment processes: one train used activated sludge (no nitrification), whereas the other train used advanced biological treatment (nitrification/denitrification). Samples were collected during a wet/cold season and a dry/warm season in 2004, and once more in a second year (2005). The two sampling events in year 1 were based on hydrology and treatment considerations. In the summer, river flow is low, so some streams are more effluent-dominated; and, there is more nitrification at the WWTP. In the winter there is more flow and less nitrification. These two seasons showed the different impacts of hydrology and treatment. In year 2, many of the utilities were re-sampled in the season that provided especially informative data for that system to ascertain temporal (year-to-year) variations. Includes 9 references, figures.