This chapter examines accuracy of data on characteristics and behaviors of network ties by assessing concordance between 327 indexes' reports on their 411 network members and the network members' self-reports. Participants were recruited for an HIV prevention intervention. Almost all (99%) participants had a history of drug use. Concordance rates varied by the type of information reported and raters' characteristics and behaviors. Overall, substantial concordance (kappa = 0.68) was found for reports of network member's HIV status, with sex partners having greater concordance (kappa = 0.80) compared to HIV seronegative and current drug using indexes (kappas ranged 0.55-0.61). Yet, participants generally, and HIV seronegatives especially, tended to underestimate HIV infection of their network members. Reports of network members' current drug use (kappa = 0.45), current injection drug use (kappa = 0.58), and employment (kappa = 0.52) showed moderate concordance, and age showed near perfect concordance (kappa = 0.90). Results suggest that former and current drug users can provide reasonably accurate data about the HIV status and drug use of their network members. Implications to HIV prevention are discussed.