Microbes were the first organisms to evolve on Earth, and inhabit every known environment, including the human gut. Diverse microbial ecosystems are involved in a number of crucial bodily processes including educating the immune system. Interestingly, human microbiome composition and function is also linked to central nervous system activity, specifically, the stress response. Extant literature exploring the stress-microbiome connection in healthy, human adult samples is sparse and primarily tests these links in non-human animals or in clinical human samples, particularly those with depression and anxiety. This dissertation is among the first to explore stress-microbiome links across three stress domains in two samples of healthy adults, specifically, perceived stress, stressful life events, and cardiovascular function surrounding stress as indexed by Respiratory Sinus Arrhythmia (RSA). This vital next step will help researchers better understand the nuanced, dynamic connections between stress types and microbial composition.We carried out two main studies: Study 1 includes 62 healthy adults, 68% female with a mean age of 37.3 years and Study 2 includes 74 healthy women with a mean age of 41.6 years. Participants completed surveys assessing stressful life events, perceived stress and completed a laboratory stressor. RSA was collected prior to, during, and following a laboratory stressor. Following the laboratory visit, participants were given fecal collection kits to collect microbiome samples that were assayed for microbial composition. In Study 1, the low perceived stress group was higher in alpha diversity than the high perceived stress group. Both Study 1 and Study 2 revealed differences in beta diversity between stressful life events groups and Study 1 revealed differences in beta diversity between RSA stress reactivity groups. Differentially abundant microbes between groups are discussed. Further, levels of Clostridium were negatively associated with RSA stress reactivity in Study 1 and levels Escherichia/Shigella were positively associated with perceived stress in Study 2. Together, these studies show that stress group membership was differentially linked to microbial composition and that objective assessments of potentially severe stressors such as stressful life events may be more reliably linked to microbial composition (beta diversity) than subjective evaluations of perceived stress. This work provides a foundation for future research that should focus on experimental studies and longitudinal interventions designed to determine bidirectional links between stress types, bacterial species, and metabolic output.