Phthalates are a class of ubiquitous synthetic chemicals found in commonly used products, leading to widespread human exposure. A growing body of evidence demonstrates the adverse association between phthalates and asthma, a leading chronic condition among children. Evidence from in vitro and ex vivo studies suggests that phthalates induce pro-inflammatory mediators and other immune responses characteristic of asthma, and evidence in animal models suggests phthalates may act as adjuvants in allergic airway reactions, with some support for oxidative stress as an underlying pathophysiological mechanism. Epidemiological studies suggest phthalate exposure is associated with asthma outcomes, though the majority of studies do not investigate the role of phthalates as asthma triggers in children diagnosed with asthma. This dissertation reports on a study that accessed participant data from the Home Air in Agriculture Pediatric Intervention Trial (HAPI) to examine longitudinal relationships between phthalate exposure, respiratory outcomes, and biomarkers of oxidative stress in a cohort of children with asthma. In 2015-2019, participants provided urine specimens and completed comprehensive health outcome assessments at up to 4 individual time points over a year of follow-up. Collected at these time points were exposure measures for 11 urinary phthalate metabolites (plus one summed measure, ∑DEHP) as well as outcome measures for fractional exhaled nitric oxide (FENO), urinary leukotriene E4 (uLTE4), spirometry, the Asthma Control Test (ACT), a caregiver questionnaire of perceived symptoms, and urinary biomarkers of oxidative stress (a biomarker of lipid peroxidation via measure of 8-isoprostane and a biomarker of DNA/RNA oxidative damage via combined measure of 8-hydroxydeoxyguanosine (8-OHdG), 8-hydroxyguanosine (8-OHG), and 8-hydroxyguanine). Laboratory analyses on urine specimens to quantify all biomarkers of exposure and biomarkers of effect were carried out through the Children’s Health Exposure Analysis Resource (CHEAR). We utilized linear mixed effects models and generalized linear mixed models to estimate associations between urinary phthalate metabolites and all outcomes. Our primary models controlled for specific gravity, age, sex, the HAPI intervention, body mass index (BMI), atopy, and season. We investigated effect modification of FENO by atopic status. A total of 79 participants provided 281 observations in this study. Creatinine-corrected urinary phthalate metabolite concentrations were comparable to levels reported from National Health and Nutrition Examination Survey (NHANES) data (2015-2016). Geometric means (geometric standard deviation) of phthalate metabolite concentrations ranged from 1.4 μg/g creatinine (2.5 μg/g creatinine) for MEHP to 25.4 μg/g creatinine (2.5 μg/g creatinine) for MEP. In linear mixed effects models, FENO concentrations generally increased as urinary phthalate metabolite concentrations increased, although not all associations were statistically significant. For a doubling (100% increase) in urinary MECPP, MEHP, MEP, and MNBP, FENO increased by 7.9% (95% CI: 0.7 – 15.7), 6.4% (95% CI: 0.0 – 14.1), 6.4 (95% CI: 0.7 – 13.3), and 8.7% (95% CI: 1.4 – 16.5), respectively. We did not observe statistically significant interaction with atopy. All phthalate measures (including ∑DEHP) demonstrated statistically significant, positive relationships with uLTE4, with effect sizes ranging from a 6.4% increase in uLTE4 (95% CI: 2.8 – 10.2) for a doubling in MCIOP to a 17.3% increase in uLTE4 (95% CI: 12.5 – 22.3) for a doubling in MNBP. There were no statistically significant associations between urinary phthalates and caregiver report of child asthma symptoms in the two weeks prior, FEV1, or ACT score. The oxidative stress urinary biomarker, 8-isoprostane, increased as urinary phthalate metabolite concentrations increased. Effect sizes ranged from a 7.9% (95% CI: 2.8% - 12.5%) increase in 8-isoprostane for doubling of MCIOP, to 19.8% (95% CI: 13.3% - 26.6%) increase in 8-isoprostane for doubling of MNBP. The DNA/RNA oxidative damage biomarker (combined measure of 8-OHdG, 8-OHG, and 8-hydroxyguanine) generally increased as urinary phthalate metabolite concentrations increased, though only associations with MCINP and MEP were significant in our primary models. For each doubling of MCINP and MEP, the DNA/RNA oxidative damage biomarker increased by 5.7% (95% CI: 0% – 11.7%) and 5.7% (95% CI: 0.7% – 10.9%), respectively. In conclusion, in a cohort of children with asthma, we observed significant associations between urinary phthalate metabolites and concurrent biomarkers of inflammation and oxidative stress but not with clinical metrics such as symptoms in weeks or month prior or spirometric measurement of obstruction (FEV1). The most consistent (across phthalate metabolites) and pronounced effects were observed with uLTE4 and 8-isoprostane. Future studies of phthalate exposure and asthma outcomes should investigate these biomarkers of effect further, as well as the pathophysiological mechanisms they represent, in order to shed light on the role of phthalates as exacerbators of childhood asthma.