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Background: Organophosphate esters (OPEs) are a class of flame retardant, plasticizing, and lubricating chemicals that have been in use for decades. They are subject to a patchwork of bans in the United States and elsewhere due to preliminary human and animal evidence of toxicity. However, OPEs have been sharply rising in production volume since the mid-2000s, when they largely replaced polybrominated diphenyl ethers as chemical flame retardants for consumer goods, and we still do not have sufficient scientific research to conclude whether they pose a significant threat to public health. For this work, we focused on characterizing exposure to OPEs in pregnant women and children and investigating any associated thyroid hormone or neurobehavioral disruptions. Methods: We used data from the Health Outcomes and Measures of the Environment (HOME) Study, a longitudinal pregnancy and birth cohort which recruited pregnant women from the Greater Cincinnati area from 2003-2006. House dust was collected at 20 weeks, 1 year, and 2 years for OPEs. Participants gave urine samples at 16 weeks, 26 weeks, delivery, 1 year, 2 years, 3 years, and 5 years for the analysis of OPE metabolites. Thyroid hormones were analyzed from pregnant women’s blood at 16 weeks and cord blood at delivery. Child cognition and behavior were measured by direct assessment and questionnaire, respectively, at ages 3 and 8 years. We used generalized estimating equations to model associations between individual OPEs and health outcomes and mixture methods—including Bayesian Kernel Machine Regression, quantile g-computation, and latent variable analysis with structural equations modeling—to estimate how combinations of OPEs work together to affect neurobehavior.Results: OPEs and their metabolites were detectable in the homes (n = 317) and urine samples (n = 357) of 100% of our pregnant study participants. Urinary bis(1,3-dichloro-2-propyl)-phosphate (BDCIPP), and diphenyl-phosphate (DPHP) and di-n-butyl-phosphate (DNBP) more weakly, were associated with decreased T3 and T4 and with increased thyroid stimulating hormone in pregnant women and newborns. Prenatal OPEs were not associated with child IQ at age 8 years. However, BDCIPP, bis-2-chloroethyl-phosphate (BCEP), and DPHP at ages 1-5 years were associated with an additional 1- to 2-point decrease in 8-year IQ among socioeconomically disadvantaged children. Pre- and post-natal urinary OPE mixtures were associated with child internalizing, externalizing, and overall negative behaviors at ages 3 and 8 years, but the direction of effects and windows of vulnerability were not stable.Conclusions: Higher exposure to OPEs, measured by a urinary biomarker, was associated with maternal and child thyroid hormone disruptions and some alterations in child neurobehavior. As our findings are among the first on these topics, more work must be done before we can form recommendations on whether OPEs are safe for the public. Future studies should focus on frequent biomarker sampling, diverse populations, and additional study designs.