Estrogens are a group of endocrine disrupting compounds capable of causing abnormalities in the reproductive systems of the wildlife. Wastewater is a major source of environmental estrogens, in part due to incomplete removal of estrogens in biological wastewater treatment processes. This dissertation investigated factors affecting estrogen biodegradation in bioreactors. Specifically, research efforts were placed on characterization of several bacterial estrogen degraders (model strains: Aminobacter strains KC6 and KC7, and a Sphingomonas strain KC8) and examination of the effects of operating parameters on estrogen removal and estrogen-degrading microbial community structure. Sphingomonas strain KC8 can use 17beta-estradiol as a sole carbon source, suggesting that estrogen degradation by KC8 is a growth-linked, metabolic reaction; however, estrogen degradation by strains KC6 and KC7 might be a non-growth linked, cometabolic reaction. One important finding was that strain KC8 can also degrade and further utilize testosterone as a growth substrate. Strain KC8 was characterized in terms of its utilization kinetics toward estrogens and testosterone with the results that showed relatively smaller kinetic parameters than the typical values for heterotrophs in activated sludge. Strain KC8 can also grow on other organic constituents (glucose, succinate, and acetate). Strain KC8 retained its ability to degrade both 17beta-estradiol and estrone (after 15 d of growth on a complex nutrient medium without 17beta-estradiol). Effective removals (>98.7 %) of 17beta-estradiol with no significant differences were observed in sequencing batch reactors (SBRs) under three solid retention times (SRTs of 5, 10, 20 d). The population ratios of known estrogen degraders (strains KC8 and ammonia-oxidizing bacteria (AOB)) and amoA gene (associated with ammonia oxidation) to total bacteria decreased as SRT increased in SBRs. These observations correspond to the decreasing percentages of 17 beta-estradiol biodegraded in SBR when SRT increased from 5 to 20 d, when the sorption of 17 beta-estradiol onto biomass was considered. Real-time terminal restriction fragment length polymorphism showed that more ribotypes were observed in SBR-20d than SBR-5d. The species evenness (E) in microbial community structures in SBRs was not affected by SRT. However, diversity indices (Shannon-Weaver diversity index (H) and the reciprocal of Simpson?s index (1/D)) suggest that longer SRTs might lead to a more diverse microbial community structure.